You don't have to buy the pass from the same place as you buy your transatlantic ticket. For example, you can buy the transatlantic flights with US and then buy the pass from United, Lufthansa or even a travel agent.

One agent in the US who I know has experience of issuing these is STA Travel (www.statravel.com). They are primarily an agency for students but they will deal with anyone. They also have an office in Cambridge (65 Mt Auburn St Cambridge, MA 02138 617-576-4623 )
. All they or anyone else needs to issue the pass is the ticket number(s) for your transatlantic flights. They will then cross-reference the bookings so that the carriers you fly with can verify you have a transatlantic *A flight. If you encounter any confusion just make clear to them that the fare you're looking for is the Star Alliance European Airpass (I have found that bringing a printout from the Star Alliance website can be helpful to staff in isolating the precise fare you want). Alternatively just tell them what you want to do and see what suggestions they come up with.

Caveat: I have never used their call center but have made quite complicated bookings in person at their offices in DC and New York. I've had universally good experiences (though the more junior staff seem to refer to higher authority on a lot of things).

Edited to add: you only need 3 flights on *A to get the pass fares. You could always add on AGP as a side trip using a low cost carrier from Germany, France or Spain. I would book the LCC flights (or make sure they're available at a good price) before finalizing the pass.