I don't entirely expect this to be answered but let's give it a shot.  The orchestra credited in Orchestral Game Concert 2 is the Tokyo Memorial Orchestra.  Out of curiosity I googled for more info both in English and in Japanese (東京メモリアルオーケストラ) and came up with little more than, well, OGC2 references.

Was this a special orchestra that was commissioned just for the album?  An older orchestra that was renamed or no longer exists?

I think how this works is that it's made up of individually hired "session players", so that talented players are there but it keeps the costs lower compared to using a well known "brand name" (and higher cost) orchestra.

I know plenty of album productions are done that way but isn't that pretty uncommon for a live concert?

Nope, it's just as prevalent and applicable for live performances as it is for studio/album performances.