Forwarding via different external networks

[Paul Kosinski]

I have a rather unusual network with a gateway machine that connects to
two ISPs: a slower DSL with a static IP and a faster cable (Comcast)
with a DHCP IP. The gateway machine runs two instances of BIND (plus
the usual firewalling): an authoritative one for a couple of domains
(and only those domains!), and a forwarding-only one for use by internal
clients (to reduce external DNS requests via the usual caching that
BIND provides).

Currently, I forward all outbound DNS via the DSL to the ISP's DNS
servers. (I have more confidence in the DSL provider not interfering
with DNS than in Comcast.) However, there have been a couple of cases
recently when the DSL was not getting beyond their gateway router,
which meant that DNS would fail, causing much HTTP(S) to fail even
though the cable network was working quite nicely.

So my question is, is it possible to configure my forwarding BIND to
have a primary and *secondary* path for sending out DNS queries? As far
as I can tell, the "query-source address" option in named.conf only
allows one outbound interface to be (implicitly) specified, and I don't
want to leave the outbound interface unspecified as that would defeat
monitoring and logging on the specific interface. The "forwarders"
option *does* allow multiple DNS servers to be specified, but that
doesn't help if the network path is down.

P.S. I suppose I might try something with policy routing, but that was
already a nightmare to set up, since I use DSL vs cable based on the
source and type (e.g. HTTP, SSH) of the traffic rather than the more
common destination.