I wonder if the empeg/riocar could be hooked up as a CD changer to the "infotainment" stereo?

This will almost certainly not work.

In Saabs with the previous dash style (mine), only Saab OEM CD changers will work. This has to do with the head unit using a proprietary protocol (which I think should be defeatable), but primarily because the CD Chnager would have to be "married" to the head unit by a dealer. The empeg lacks the electronics to be married to the HU, and I have yet to see a device that can fake-out the HU into thinking it's a Saab changer.

I have driven a couple of the new 9-3s. Both of them had the in-dash 6 disc slot changer. It is seperate from the main HU and sits lower on the dash. I think you may be able to fashion a DIN slot out of that pretty easily.

If you get the new 9-3, I would get the most low-end stereo you can then rip it out completely. For instance....

My Saab came with some external amps from the factory. They're still in the car, but nothing is running off them because they won't work with aftermaket stereos. Speakers also had to go since they started blowing out after using aftermarket amps for a couple weeks.

The lucky thing about the older 9-3s is that the dash is double DIN if you can live without the cup holder. And I think I can make it triple DIN if I toss the SID (orange dot-matrix display in the center of the dash). I have GPS/DVD HU plans for my Saab right now. Plans that would probably never come to light if I had the new style 9-3.

Anyway... The chasis on the 9-3 is excellent. Far superior to the old 9-3 (including the Viggen). Go for at least the Arc or Vector models. Linear has no balls IMO. If you are also looking at bigger cars with lots of power, you shouldn't pass-up the 9-5 Aero. Plus the dash in the 9-5 should be easier to work with. New 9-3 is hot though. Especially the convertible.