Well, after speaking to several install shops today, here is what the final tally ended up on installation methods.

2 shops wanted to use line level converters. Apparently that's common practice with Bose systems according to these two shops.

1 shop wants to just use a harness with line inputs and wire it up direct with no interface of any sort

1 shop says it can't be done and I have to replace everything.

Given that I guess I'm going to go with my original option and see how things turn out. I searched valiantly for a shop here in my neck of the woods that used the OEM-1s but the only shop I found that even had one had no harness for it and had never used it.

Crossing my fingers, I guess we'll find out tomorrow.