Yes, there's now a /net/ipifc/xxx/snoop.  If you point to
that rather then /net/etherxxx, you can snoopy packet
interfaces.  However, you don't see the ppp messages,
just the IP that are the contents.

If you want to see the pre-ppp processed data, you can
stick something twixt the serial interface and the
ppp process, or turn on debuging in ppp with the -d
option.