I'm comfortable calling the Radian incompatible RF behind the passenger seat in my 2005 Mazda 3 for an older kid without the angle adjuster. With the angle adjuster it could be compatible, but I still can't get enough room in the front seat for a taller passenger (I'm 5'8" or so). I've never had luck in the middle at all, but haven't tried with the angle adjuster. As PP said, I do fine bracing it on the driver's seat without the angle adjuster and with the angle adjuster I think it would accommodate a taller driver than me as well.
What is it that you like about the Radian? If you give us your baby's height and weight and some of the things you're looking for perhaps we could brainstorm some alternatives?
ETA: I just read your other thread. Frankly, for a child of normal size / weight, and if you're OK with it not being the last seat you buy, I would not buy a Radian for the Mazda 3 even with the angle adjuster unless you only have very short adults that ride in the front seat or two non-tall drivers that drive at the same position. If it were me, I'd probably get a MyRide for now, knowing that it's significantly cheaper than a Britax, my kid liked it, and it will be outgrown at the same height as a Britax. The other option might be a TrueFit, though I've never seen it in this car. It has a tall shell but only rear faces to 35 lbs. Britaxes have fit fine in this car for me.
Now, take my advice with a grain of salt: we are very big and tall people (mom: 5'8"+, dad: 6'9", baby at 24 months was 36 lbs and tall) in a small car. The recline on the Radian in the Mazda is way harder to deal with with a heavier child: he squishes down the seat much more in this car than he does in the other cars I've had it in, with and without the angle adjuster, and the fact that you can't touch the passenger seat is much more of a deal breaker with a larger RF kid than it was with an infant (who does not climb in and bounce and push his feet against the seat). But if I had a Mazda 3 and I didn't need a Radian to get my kid RF past 2, I wouldn't have one. I used the MyRide up to its limit to avoid it (again, this is pre-angle adjuster).