While the idea is not entirely without logic, speaking as a third year medical student, I'm not convinced your understanding of the anatomy is entirely correct.
The muscle responsible for shrinking things pre-op is the cremasteric muscle, which is no longer there. All you now have is a donor skin graft on to an internal cavity, yes it is a full thickness graft, and comes with vascular and nerve connections intact, but there is no muscle. The lowest layer taken is the dermis, so the tissue won't behave in eaxctly the same way.
HOWEVER - that said any skin would tend to soften and slacken when gently warmed, as the warmth could affect the collagen, so your idea is not completely without merit, just not for the reason that you gave, although as your body cavities are already pretty warm, I'm not convinced that effect would be dramatic. In other words if it works it does so for a diferent reason than you might suppose.
Oh and one other very important thing... warmpacks - NOT hot please! I get to clerk enough weird injuries in Accident & Emergency without having to deal with someone who has scalded her neo-vag! Please remember that while gently warming tissue slightly might help, heating it well beyond normal temperatures will only inflame it, make oedematous, possibly scald or burn it, and really wont help at all! So be careful.
