domain mapping category of base site as new domain root

Having a hard time easily asking what I need, which also makes it difficult to search for this, hope it hasn’t’ been asked.

I’ll tell you what I have, and if someone can tell me if 3.0 MU can do this for me I’d be greatly appreciative.
I did this through a stack of plugins a few years ago on a standard wordpress install (not mu)

I have my main website:
https://www.gtluke.com

I have some offshoots of my site I use for business use.
Each site is actually just a category of my main website.
https://www.rallyaddict.com <- is actually https://www.gtluke.com/rally
https://www.notights.com <- is actually https://www.gtluke.com/biking

3 different “sites”, 3 different “themes”
1 wordpress install, 1 post goes to multiple “sites”.

If I photograph a bike race, I just make my post in gtluke.com and put it in the category biking. Now it appears on my main gtluke.com site and on my notights.com site. Though you can’t actually tell this is happening. Keeps my rally and biking business visitors from browsing my vacation posts on my main site, but I only have one actual site to maintain which combines them all.

I find this awesome. But I can’t seem to replicate it’s design in 3.0
I don’t want to nuke my site to 3.0 to try it out. I installed and replicated my site in https://www.gtluke.com/wordpress (someone root died after multisite enable, haven’t investigated yet) but since it’s not root, the domain mapping plugin will not function, so I can’t try it out.

Oh, and how bad is this going to destroy my existing links? I have hundreds of links to my site around the net, any way i can keep those?
It would have to stay the same structure I had it, gtluke.com/category/post

Before I destroy my site(s), can someone point me in the right direction here? Thanks!
-Luke