Proxying requests based on regexp pattern

Hey there :)

I'm working on this project, building an online store in Ruby on
Rails, which is to replace an existing, very old, store written in ASP
running on an IIS machine.

When launching the first release, some functionality for specific user
groups remains on the old server, while the majority of the site will
be running off an Apache-controlled Mongrel cluster (via mod_proxy &
balancer) on a new (Windows) server.

I'm now wondering about how to approach this problem, getting the
legacy requests routed transparently to the old server while having
the Mongrel cluster working happily with the new requests.

Luckily (I think), everything on the old server is served by one entry-
point page in the root of the site, called default.asp, and no page in
the new site should ever get such a name, so I'm immediately thinking
regular expressions to detect and proxy out requests that should go to
the old server.

I'm just not sure where to go next. VirtualHost seems clear to me to
not be of any use, I'm not sure about mod_proxy, and I don't think
mod_rewrite is the deal either. Maybe some kind of HTTP header? Frame?

I also have to consider that there may be incoming SSL connections,
POST data, etc.

Basically, I'd like to hear if any one has tried this kind of setup
and get some pointers as to where to go from here.

Thank you very much in advance,
Daniel Buus :)