I've been able to find no examples online of anyone sending requests from a master for the return of specific data from a slave.
I agree with this as stated, however, just for other's future reference, there are 6 short example sketches in the examples folder under the wire lib. Four of these are the various combinations of master and slave comm - but none for an entire comm loop. I only mention this because it seems that lately many people are not aware of the examples in the lib folders.
bigfun, I understand your situation. I've had similar problems. What I've come to believe is that response from the Slave must be almost instantaneous after the Masters request. (I remember one project I had where the slave would immediately respond to that master with a value of how long it must wait until making the request again.)
I also know about "clock stretching" on the Slave side but haven't tried it. It might be simply a matter of the slave setting the clock pin low with a digitalWrite.
In any case, I'm interested in how this thread develops, and what Nick has done.