Well, to answer your original question: no, it is not supported. It should
be gracefully rejected - but I bet that part is more or less untested ;-)

If you send my tcpdump -vvv output from the the ethernet interface used,
or "ifconfig pppoe0 debug" output off-list, I'll probably be able to 
tell you what goes wrong. If PAP is involved, be sure to nuke username
and password in the trace ;-)

Martin