You could always just 'roll your own'...
It will require a bit of investment in terms of time and learning... but it is actually relatively easy to make your own photoreal sceneries... and the tools are all freely available... hell if I can do it, anyone can!

and as long as you're not distributing it, I dont think google are going to complain

It may not look as good as the stuff Robin puts out (but as long as you can live with the odd shadow here and there, you can actually turn out large chunks of photoreal pretty quick... the only reason my FoxpineX took a while, was because you tend to fly around airports quite low, you need to get rid of the shadows etc...
For general countryside, from a couple of thousand feet, you're not really going to notice too much... and I am fairly sure it will look better than the default FSX!
Thanks for the heads up. I have been looking at Tileproxy and it looks like the way to go at the moment. Do you have any other suggestions to start me off?