Is it possible to arrange routing such that a gateway machine speaks over
   one ppp link when the packets originate on the machine itself, and over 
   a second ppp link when the packets are being forwarded from some other
   machine on an internal network?
   
   Ie: may I select a network interface based on the originating ip?

i can't think of a way to do this with 'normal' tools...

but, i think you could work something out with the ip-filter package,
using the ipnat program, by routing the incoming network to one ppp
interface, and everything else to another.  doesn't seem too hard to
work out, but i'm only guessing  :-)

see ipnat(1), ipl(4) and pfil(9) for details.