Since you mentioned that the system was embedded Linux, see in particular Chapter 11. Some combination of
packet marking and policy routing should get you taken care of.

Good luck,

Christian Gagneraud wrote:
> Finally, my problem is that i would like to simply force the reverse
> tunnel to use only ppp0. And at the same time i need the default route
> to go through eth0 (that is needed for the main programs running on this
> box)