@cocreature pointed out to me that I may have misread your question. Is this what you are after or are you looking for a way to restrict what parties that can go into the signatory
clause?
The latter is possible, but I’d consider it a hack. There’s a discussion of the topic here. TL;DR:
import DA.Optional
template T
with
sig : Party
where
signatory sig
ensure sig `elem` mapOptional partyFromText ["Jean", "Pierre", "Paul"]
It’s a hack because you are hardcoding the string party identifiers into the template. That will make the template highly non-portable.