Since you mentioned that you already have some devices (keyboards) that produce "auto accompaniment" and "rhythm", you must realise that you already have one or more "arrangers", regardless if they accept or transmit MIDI or not.

If I understand, the problem is to "control" those MIDI-less things from an independent, external source.

I don't think that it can happen easily... but why don't you list what you have (makes, models) and what you want to do (e.g. keyb 1 accompaniment + keyb 2 rhythm +keyb 3 right hand=live music) so that older members can contribute.