[Gregstrom]

It's time-consuming, but... If you gave each nation a series of spells that could summon unique researcher units (so multiple spells and unique units would be required for each nation) then you could both cap the number of researchers per nation (i.e. if a nation had 5 summon spells, they could only get 5 researchers) and, by setting costs and/or research levels of the spells appropriately, you could limit how quickly a nation got them.

Inanimate units may well be immune to seduction, I think.
Complete immobility might be a good idea, as it makes them less usable in combat.