Hello alcalc,
I have, word for word, the exact same problem. For the past 5+ years I have, on and off, been troubled with this problem. I am not convinced there is a simple method to achieve this, the motion platform is constructed from 6 'actuators' that push the top platform around, that in turn, is restricted by universal joints. This is easily achievable when controlling the individual actuators but becomes a crazy 'cyclic' relationship when attempting to control it via a central IK.
I wish I could offer some actual, practical, advice but I am actually writing to find out if you managed to find a solution.
Please let me know, maybe between us we can get a semblance of something workable.

Cheers