Have you thought about using these?
http://www.seelectronics.com/IRF.html
- at least in addition to some rough and ready, removable wall treatment, moving the piano etc.
I've
heard the vocal version is actually very good and a lot less gimmicky than you might think. I'm going to be borrowing one soon to try out - they're not exactly cheap. Not heard so much about the instrument ones and certainly not when used on such a large and unique instrument, namely a grand piano.
Also I was thinking (for my own use) that they (or rather the vocal one) may be great at homoginising (sp?) as well as just improving the sound of various (practice/ demo type) recording sessions (to be edited together), spread out over time and possibly recorded in a variety of differently unsuitable spaces.
For your situation, if they'll fit your mics and help enough (in addition to... etc) then at least they are small and probably useful thing to have ....
I'd be very interested to hear about anyone's experience with the vocal version (perhaps on another thread).