I believe it IS indeed legal to put it back together. The rules regarding the demil process basically are written to make it difficult to easily re-assemble the demilled item. Note I said "difficult", not "impossible". Granted, making a 100% operational tank with functional weapons is illegal, but a driving tank is fine. The DRMO knows that no foreign government is going to remilitarize these tanks. Rewelding them back together may make a driving tank, but it will never again hold up to the rigors of combat, plus a 3rd world country doesn't want an Abrams. They want a T-55 or T-62, something that is easy to work on, easy to find parts for, and can be easily driven by 4th grade educated 3rd world rejects.
If you luck out, be sure to post LOTS of pics, Joe!