Obviously if someone has done that already that'd be ideal.
Otherwise, I'd be happy to help along: if you provide some sample citations for different census records you have, we can figure out together how to input them into Zotero, I know both the field mapping & Chicago style very well.

Frank has done something like that very rigorously for legal citations for his MLZ version of Zotero - there is no reason not to do that for items common in genealogy itself.
http://citationstylist.org/proofsheets/
If you want to go that route (and I think that'd be cool), ideally you'd find some collaborators interested in this, as it's an insane amount of work and I'm not going to be an active participant beyond helping out if you need technical advice.

(New fields/item types are also not completely out of the questions in the medium run if they're really needed & now would be a good time to find that out - what's not going to happen is a proliferation of item types to match the different types mentioned in EE)

(I haven't forgotten this, but there are some issues dealing with archival material according to CMoS in Zotero/CSL - see e.g. https://github.com/ajlyon/zotero-bits/issues/27 so this may require more groundwork on the development side before it can be done properly).

(I suspect it's from Turabian manual which in it's last edition still follows CMoS 15 as far as I know)

On June 30th, Kulath wrote:
>EE has the URL in brackets, and the 'accessed' item after the URL. The Chicago summary http://www.chicagomanualofstyle.org/tools_citationguide.html) has no brackets. The order of the 'accessed' item changes between CMS 15th and 16th edition. Where do the brackets for URL in EE come from?

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Kulath, I have just now seen your question. Thanks for asking it. FYI, EvidenceExplained.com also has query forums where questions about citation, evidence analysis, or record usage can be asked directly for much quicker answers.

Short answer to your question
EE’s format for citing material published online follows the traditional format for citing material published in print. This is one of the core requests made by both programmers and researchers while EE was under development.

Before I give you the long answer, I should clarify one point: EE uses parentheses for the function that you question — not brackets. Among historical researchers, expository writers, and editors, parentheses and brackets have quite different functions.

Long Answer: A Comparison of Templates

A basic format for citing a published source would be “Basic Book” (aka monograph: a book with a single focus and the same author for all its content). Using this, let’s compare the traditional reference-note format (whether EE, CMOS, Turabian, or whatever) to EE's reference-note format for a simple website publication:

Traditional book citation
Author/Content Creator, Title of Book (Publication place: *Date), page/figure/whatever.

EE website citation
Author/Content Creator, Title of Website (Publication place/URL : Date), page/figure/whatever.

*Most style guides give us the option here of citing (a) Date; or (b) Publisher, Date. EE's website option allows citing whatever date is appropriate: date of access, publication, or update. A field for publisher is rarely necessary when citing a website.

To extend this analogy, let’s compare the traditional way of citing a collected work (aka anthology, etc., in which each essay or chapter has a different author but the whole has one editor or editorial team) to a website that has many separate offerings by different creators under different titles:

Traditional citation to “collected work”
Author of Essay, “Title of Essay,” in Title of Collected Work, Name of Editor or Editors (Place of Publication: date), page/figure/whatever.

Example:
Elizabeth Shown Mills, “Marie Thérèse Coincoin (1742–1816): Cane River Slave, Slave Owner, and Paradox,” in Janet Allured and Judith F. Gentry, Louisiana Women: Their Lives and Their Times (Athens, Ga.: University of Georgia Press, 2009), 10–29.

EE citation to an online counterpart
Author/Creator of Article or Database, “Title of Article or Database,” at Name of Website Creator, Title of Website (Publication place/URL : Date), specific item cited.

Example:
Lucia Stanton et al., “The Monticello Plantation Database,” at Thomas Jefferson Foundation, Monticello (http://plantationdb.monticello.org/nMonticello.html : updated 2008), for “Isaiah Male 12/31/1800- (ID: 407).”

Within this framework, essentially all the other standard rules for print publications apply to online publications. For example:
• If there is no identified author/creator, the field is left blank.
• If there is no editor for the collected work (or the creator of the collected work is the same as the title of the website) the unneeded field is left blank.
• If the book exists in multiple editions or has multiple volumes, an extra field is needed to state which edition or how many volumes. Similarly, EE's website citations use that field to identify the type of online material being cited (i.e. “database” or “images,” etc.), if the title of the article or database did not indicate what type of material it is.
• If the cited source provides a citation for its own source and that needs to be recorded, it can simply be added to the end of the citation.
• Etc.

Kulath also stated:
> ESM seems to claim that her work is derived from CMoS (not Turabian) (though I can't find a reference just now).

Kulath, I’ve addressed that point in a separate thread at
https://forums.zotero.org/discussion/21137?page=1#Item_6

Questions?
Elizabeth Shown Mills
www.EvidenceExplained.com