United States State Capitals are encoded with feature code PPL. It seems to me that they should be marked as PPLG, or perhaps some other distinguishing code.
The same would apply to the administrative seats of ADM1 level entities in other countries as well, assuming such exist.
I would be happy to provide you with the names and GEONAME IDs of the US Capitals.
You are right. The Capitals of US states should have the featureCode PPLA for seat of a first-order administrative division.
I will update the datebase if you can send me the geonameId.
I try to run most updates through the normal modification chain to add a history and include it in the daily modification dump. Sometimes however the database is directly updated without including the modification in the history and the daily modification dump.
I can start a forum thread with these updates, so you can repeat it on your geonames copy.