[The APA] however suggest the ever elegant "Manuscript submitted for publication" or "Manuscript in preparation" in later individual examples. (p. 211)
... "in press" is obviously a valid status that is distinct from "forthcoming." Parenthetically: I do not think "in press" is misused any more than "forthcoming" is.
Write in press in parentheses for articles that have been accepted for publication but that have not yet been published.(p. 185)
As far as APA is concerned in press is the only thing (other than n.d) that should be substituted in the date spot.
To follow on from Bruce, perhaps this could this be handled by a single boolean toggle in the input, and an assortment of terms.
If an item has 1) a date of publication in the future 2) the word forthcoming or 3) the words in press; in the date field...