I'm experimenting with compressing the POST data on the client side using LZJB algorithm. In my tests it took the POST size down by about 50%.
It's available in the latest sandbox build (6.2.2165).
If you include the iuppiter.js library (also included in the js directory of the demo) in the page with the Scheduler the callback requests will automatically be compressed. It seems to be very fast (no delay is noticeable) and quite effective.