I have ISDN set up to dial-on-demand, with a default route set up through
ippp0.

However, when the link is taken down after the huptimeout expires, the
default route is also deleted, effectively disabling the dial-on-demand
functionality.

I tried removing the 'deldefaultroute' config option. I tried removing the
'defaultroute' option. I tried adding ippp0 as the GATEWAYDEV in
/etc/sysconfig/network.
Eventually it appears to work now I've also added an IP address as GATEWAY
in the same file.

isdn-config ought to do this for you, and it ought not need a static ip
address as the remote end of the link.

Also, why does it unconditionally disable all forms of compression? I've
edited the /etc/init.d/isdn script to 'modprobe bsd_comp' instead of adding
options to the ipppd command line to disable it.