Sorry for my abruptness :) Sometimes I forget what it's like being a new user and not knowing the backstory on these things.
Basically, it was a big decision to move to the 10.6 SDK, and a lot of work was involved, too. Not only does XBMC now require many things that are only in the 10.6 SDK, but a lot more things have been changed to correctly work with that SDK.
Is it physically possible to do? Sure. It's just a really big task, and I really doubt any of our OS X devs would do it. If they were up to the task, then we'd probably still be supporting the 10.4 SDK.
The best thing to do in this case is to take Linux for a spin. I have a few Apple TV 1s, and these first gen ATVs run a modified OS X 10.4. However, they now run linux on them, and not only does linux typically run better on older hardware like this, but it allows them to use XBMC v12.
I'd go and try out
OpenELEC (wiki) and put it on a USB stick. That way your internal OS install isn't touched, and you can test drive it to see if it allows Linux/v12 on Mac Mini to fit your needs.