makaio has done it. i dont think you should need to line it with bricks, just slope the walls...
A park near me has one.. its sick to bike on.
I could see it benefitting you since if you miss the trampoline thats above the ground you have an extra 3ft fall, where as inground saves you that. Plus it would look cool if someone see you, looks like your jumping a good 7ft on flat land
I DID IT