Ah, I misunterstood then. That's not what I had in mind.
This should work:
Insert a row for each wonder into the BuildingModifiers table with the same modifier. Use EFFECT_GRANT_UNIT_BY_CLASS with COLLECTION OWNER.
Set OwnerRequirement to REQUIREMENT_PLAYER_LEADER_TYPE_MATCHES with an argument checking for Shapur. Use the other requirement as before.
Since all wonder effects only take place when the building is finished, there is no reason this shouldn't work as intended. It's basically just a wonder effect, only it's the same for each wonder and only happens for one leader, which effectively makes it a leadertrait.
I realize it's not a perfect solution, but it might be the easiest way.