Bachus,
Don't know if I understood all of what you want...BUT. Today...on my PA2XPRO I can setup any external voices (SW or HW based) as a user voice.
I actually key in the bank/program chg in the Korg user voice. It will then be used like any other internal voice would, Songbook, performances styles etc. Anything.

Multiple user voices on different MIDI channels are supported. It is all controlled using a user voice. You can even use internal and external sound on the same voice...meaning I could have a Korg String AND a VST Piano on the same user voice.

As to control, there are progammable slides on the PA2 that can b set to send any MIDI CC.
Or, just set a KOrg USB based micro contrller on the kbd top.

Lee S.