The style does this already:
https://s3.amazonaws.com/zotero.org/images/forums/u452233/rfzu6iksyktfan7ynrwi.png

@aborel, @damnation
Here's an example of an RIS "TY - SER" file describing a Government of Canada publication, and below, the Elsevier - Harvard citation where I can't seem to get the report number to show. I've placed the report # "8848" in a few fields (suffixed with SN or VL, just to give me a visual as to which is being used) to test and see where that style will pick up the number, but with no success:
TY - SER
ID - https://ostrnrcan-dostrncan.canada.ca/entities/publication/b682fbf8-af0c-40b5-a243-e84070981376
T1 - Datasets to support geoscience language models
A1 - Raimondo, S.
A1 - Chen, T.
A1 - Zakharov, A.
A1 - Brin, L.
A1 - Kur, D.
A1 - Hui, J.
A1 - Burgoyne, S. L.
A1 - Newton, K. G.
A1 - Lawley, C. J. M.
VL - 8848VL
SN - 8848SN
Y1 - 2022-05-12
PY - 2022
DI - https://doi.org/10.4095/329265
DO - https://doi.org/10.4095/329265
L2 - https://ostrnrcan-dostrncan.canada.ca/entities/publication/b682fbf8-af0c-40b5-a243-e84070981376
PB - Natural Resources Canada
T2 - Geological Survey of Canada, Open File
LA - English
DB - NRCan Open Science and Technology Repository
ER -

Raimondo, S., Chen, T., Zakharov, A., Brin, L., Kur, D., Hui, J., Burgoyne, S.L., Newton, K.G., Lawley, C.J.M., 2022. Datasets to support geoscience language models, Geological Survey of Canada, Open File. Natural Resources Canada. https://doi.org/10.4095/329265

The use case is researchers can export RIS files from our repository, import the files into Zotero, create a bibliography and select the Elsevier - Harvard style, and have the report numbers show up in that citation style.

Zotero will import SN for both GOVDOC and RPRT as report number. RIS specifications have Report number from SER in C2 (???) I haven't checked, but we actually might import that.