Buy an 88-note controller, a PSR 2100 and a midi cable. Or a pro-level digital piano and the Ketron Midjay.

I had the same thought, that nothing existed that would fit my needs, but it wasn't true and I now have two stage rigs:

Rig #1: Yamaha 9000 Pro (with PLG150-PF for piano w/64-note polyphony) with a Radio Shack Piano as a controller. Works great and sounds excellent. I thought it would be inconvenient to go back to a dual keyboard rig but it isn't... the Radio Shack piano (rebranded Casio CDP-100) is superlight to carry and the action is terrific.

Rig #2: Kawai ES4 w/Music Pad Pro v4. The ES4 piano has 96-note polyphony and is a great instrument with no shortcomings. The Music Pad Pro can play back MP3 files for accompaniment and I use it with acoustic pianos as well. I considered the Ketron MidJay as an arranger and MP3 player to go with my piano but I use backing tracks more than arranger functions live and I needed a convenient way to display my music anyway so the Music Pad Pro was the way to go.

I tried everything in the under-$1000 digital piano market and found that it was all useless for me - 32-note polyphony would never cut it - but for less than twice the price of a $700 Casio or Yamaha I could have three times the polyphony with a Kawai ES4 and own a piano that would be great to play for many years to come no matter what technology comes along. The arranger part is secondary and there's many ways to address it if you really need it.