Thanks to all who have sent mail.

It appears that what I thought I wanted to do is EVIL and STUPID.

The reasonable alternatives are:

(1) make the name of the ppp interface the same as the canonical name of
my host (which requires asking my ISP to change it, which is a hassle
because they tend not to cope all that well with out of the ordinary
requests).

(2) have my ISP use an address from my class C block as the ppp address
(which I had actually suggested, since they recently went through the
dynamic IP spasm thanks to running short on addresses, but they never
responded to the suggestion, probably because they tend not to cope all
that well with etc...).

(3) use a router.

(And the fourth option, "quit worrying about it", which I think I'll do
for now...)