I looked at doing this since I have some Z3 TA's and I was thinking about using them on my TI. The TI and e30 rear both have very similar (if not the same) track width, so the e30 crowd has paved the way on this information since one of the things to do with an e30 is a 5 lug swap. This is the main reason why the TI rear sub frame is wanted for the e30.
I can say that what I found is it will take some fab to make them work but it can defiantly be done. I worked about 2/3rds of the way through this project and then I bit the bullet and bought a set of MZ3 TA's for my car instead just because that was the easier route for me since I don't have a lot of time to do the fab.
It breaks down like this:
Hub Flanges - The hub flanges that are on the Z3 are wider than the ones on the TI. This sets the offset of the hub 1 3/8" wider (per side). One possibility that I have looked at that was promising in terms of offset was to use the hub flanges from an e36 on the z3 TA with custom hubcentric spacers between the hub and rotor to set the correct depth of the rotor (I actually made these).
Caliper Lugs - The caliper mounting lugs have to be shaved off and new caliper brackets fabricated and welded on.
Parking Brake- (If you don't care about parking brake then ignore this section) I think having a parking brake is important so I was going to make this work. Using a M3 vented rear rotor for fitment the dust shield would have to be custom (but not too difficult) since the brake shoes for those rotors are larger. I've seen one guy who modified the larger m3 shoes to fit the smaller dust shield interface. My plan was to use the M3 dust shield and adapt it to the z3 hub.
Axles - axles would be the z3 axles.
http://www.r3vlimited.com/board/showthread.php?t=116129