Wow. I 'mostly' finished my install and came here to post and found this post instead! That's my install and thanks for all the nice comments! I posted an update with more pics and with links to my sub enclosure post, etc.
My custom XC90 install
Only issue still outstanding is that my AutoEQ doesn't work. I posted about that earlier here. My guess is that the rca extension I soldered on is not attached to the right points and the head unit is not hearing the mic . I tested Aux-In on the bench and aux-in still works in the car but there must be other circuit points when the mic is plugged in. One of these days I'll probably pull it to finish that up.
You guys get the credit for me choosing the 880PRS. I was orginally going to go with a 7800 until the 880PRS posts started showing up. I didn't even know about that model. And it is a great head unit with a 'few' annoyances!
To answer some questions:
1) The CD is below the display. I removed the motorized frame, and tricked the head unit into thinking it's always open. I then extended the face circuitboard away from the head unit using a computer IDE cable.
2) The CD is a sub assembly within the main box mounted at the four corners. I added spacers to the rear to tilt it down enough to line up with the factory slot.
3) I just glued my own tactile switches over the volvo circuit board and wired those to the pioneer circuit board. The knobs are directly wired. Had to break a few traces on the volvo board since it had a common ground for the knobs and center pushes and the pioneer had isolated grounds.
4) The hardest part was trying to get the smaller pioneer display to look good within the larger volvo opening.