There is a condition in export_descr_buildings called event_counter which can be used with the recruit_pool effect as in:
Code:
recruit_pool "You Unit Type" 1 0.2 2 0 requires factions { england, } and event_counter your_historic_event_name 1
When you use the historic_event command in campaign_script (or elsewhere), it increments an event counter with the same name by +1 and will thereby satisfy conditions in EDB like the one above. If you're working off of vanilla M2 or a mod, check export_descr_buildings (EDB) for usage examples.