Hi Lachlan.

What you are looking for is usually called "router on a stick".
Perhaps you can use some binat rules to specify bidirectional
mappings between external netblocks and the internal aliases.

Don't know why you are doing it, however.  The only time I made
a "router on a stick" was on my Cisco 2501.  Honestly, this set up
is an ugly workaround.

Cheers,
Igor.