There appears to be a multilink pppd available here:
ftp://ftp.east.telecom.kz/pub/src/networking/ppp/multilink/
(fetch the file ppp-2.3.8-mp.tgz) ...referenced from this page...
ftp://ftp.rubyriver.com/pub/jhardin/masquerade/ip_masq_vpn.html
..which has links to all manner of interesting stuff.
--
Dogs have masters. Cats have staff.