Why not do exactly the same thing with the p-trap where it is.

That is, the downstream half of the "U" would simply extend up to a higher elevation to provide for a downward slope to the stack. I can't see that being a violation of the plumbing code because, so far as I know, the code doesn't say anything about how deep the "trap" of a p-trap can be.

I expect that you could modify a COMMERCIAL brass p-trap like this one:

http://www.lyncar.com/plumbing/images/dvd/34300.jpg

with a short piece of 1 1/2 inch copper pipe soldered in between the two halves of the p-trap. I'd take that pipe to a muffler shop to see if they could expand one end of it, just like they do steel exhaust pipes. Expanded the right amount, so that the new ID is exactly the same as the old OD, then that piece of copper pipe would fit between the two halves of the brass p-trap to elevate the outlet of the trap as much as you want (depending on the length of the 1 1/2 inch copper pipe you use). If you use Type K copper pipe instead of DWV, you'd have extra wall thickness to allow for more expansion of the pipe end.

That will give you the elevation gain you need to provide a downward slope to the stack.

And, I don't see why anything there would violate any plumbing code because all you have is a "deeper" than normal p-trap.