does this seem likely to work (Factory HU into Empeg AUX out to amp to speakers?)

Yes, but you have to remember the following things:

1) The hard part is mounting the amp and running the wires to it. Amps don't fit inside dashboards, at least not decent ones. Even if they did, you wouldn't want it there (throws off heat).

2) Output of head unit is speaker-level amplified, so it will need a speaker-to-line-level converter box in order to go into the empeg's Aux In.

3) You still have to decide how you're going to mount the empeg. Getting rid of that phone? Too bad, looks cool.

Now, if you decide that you're going to go this route (get rid of the phone and run Tape->Converter->Empeg->amp) then you need to think long and hard about something:

Why are you even worrying about the factory system at all? Is it just because you can't imagine living without tapes and CDs?

Trust us, after using the empeg in the car, you won't miss the tape and CD player. Not one bit. I much prefer the simplicity of having the empeg as my only stereo. Just about every other empeg owner will agree with me here. There is nothing wrong with unscrewing the entire factory stereo and leaving it in your garage for the day you sell the car.