We ran an experiment at my office many years ago. If you bought a booklet of stamps, one spot was marked "this is not postage" or something to that effect. We decided to mail a letter using that final "stamp." We typed the address to make sure it was routed through the automated system as much as possible.

It worked.

Also worked: putting a 1 cent stamp on it; putting a hand-drawn stamp on it; putting an Irish stamp on it.

So yes - a foreign stamp might work for you.