Some of the bonded frames used 7075 (easton EA70) which can be welded but will crack. If trek welded anywhere on the rear at any time, it's a weld-able alloy. I think the main problem will be damage to the dropout from tightening the axle nut. The tubes will be fine if the material was clean when welded and only 3/4" of paint is lost. The normal "weld happy" alloys are 6061/7005
Welding aluminum really doesn't leave the part in "0" condition unless it goes over 700 degrees and is allowed to saturate the material.
It is OK to weld 7005 and 6061 to each other. In your case, it may have been a good thing as the dropout might harden a bit over time quicker than 6061.