I have a beef with many of these form letters: They all assume you are an American living in the states--or part of the military living elsewhere.

Well, I grew up on Cape Cod.  I have family there; and I travel there quite a few times each year.  Now, I am a permanent resident of Canada, although I am still an American citizen.

Where is the option for an American citizen living abroad?  Who, precisely, is my Congressional representative?  My last voting place in the states?

Perhaps you could pass these concerns onto Greenpeace?

Do you vote absentee?  You must have a state residence to do that...don't you?  

If you don't vote absentee, U.S. congress critters won't give a rat's rear what you think.

Yes, I vote absentee.  And I use my last Congressional district. But the forms that institutions like construct are not constructed for me.  They wish an address in the states.  
I use my last address in the states. If the powers that be want to cross reference a city directory, they might ignore you, but in my experience they generally don't.
My sister works as a staffer in my local Congressperson's office, and I can tell you that form letters, particularly those generated by email, are usually ignored.  In the past they might have keep a tally, but now they rarely bother to do even that.  On the other hand, individualized letters (mailed or faxed) and emails were reviewed and responses generated by staff, mostly using form letters (can't escape them!).  These days I fax a letter. Good luck!