What you're looking for used to be a lot easier when pools used PPS (Pay-Per-Share) a lot. For example, recently the OzCoin PPS rate was 0.00000217 BTC/share submitted. After the difficulty recalculate, it went down to 0.00000200 BTC/share. This new rate will last until the next difficulty recalculation, and then it will (most likely) go down even more. I don't use PPS, as I use DGM, but it illustrates the point.
Remember that a lot of the stuff you might be looking for has already been done. http://bitcoin.sipa.be/ has some good charts, where they estimate the network hashrate based off the rate of solved blocks at any given difficulty. They also do a good job at helping you discern between true growth and just a string of good luck. And finally, the bottom two charts show the daily growth per day. I would say 1% is a good estimate.
Thanks for your explanation, I've a much better understanding now. It does not seem realistic to me that the growth will continue to be exponential since there are human factors (eg. how quickly ASIC rigs can be deployed) so I will use a combined approach, attributing some of the current growth rate to a compound element and some to a fixed element.
Anyway, you've certainly earned the bounty. I have sent you 0.1 BTC to the address on your footers (TID e7cc6796b639ead412827238da9512836eed55f4327d1478f890b71224b9c3e6). I would be grateful if you would give me a trust point in exchange, but of course do not feel obliged.
Kate.