[Ron Ramirez]

You must be referring to one of the following Philcos which have an escutcheon/grille made of Tenite, which I also call "The Incredible Shrinking Plastic":

41-81T, 41-85T, 41-221C, 41-221CI, 41-226C, 41-841T, 41-842T, 41-843T, 41-844T, 41-851T, 42-842T, 42-843T, 42-844T, 42-853T, 42-854T, 42-PT94, 42-PT95.

These all share the same escutcheon/grille, although the colors vary. Most are swirled brown; one is solid brown (41-226C), two are ivory (41-221CI, 42-PT95).

You can see these models on the following pages:
http://www.philcoradio.com/gallery/1941a.htm
http://www.philcoradio.com/gallery/1941b.htm
http://www.philcoradio.com/gallery/1942a.htm

Philco made two models after the war that use the same grille, but in ivory only:
47-204 - http://www.philcoradio.com/gallery/1947.htm#b
48-206 - http://www.philcoradio.com/gallery/1948a.htm#f

The 47-204 and 48-206 grilles were made of an improved version of the plastic that has not shrunk or warped over the decades as have the grilles from the prewar Philco models.

I tried to get the two guys who are well known in the hobby for making reproduction plastic parts for radios to reproduce this grille since it is used on so many Philco models including the very popular 41-226C "Sled", but they both said no. Now, Phorum member OldRestorer is looking for a 47-204 or 48-206 for a grille to attempt to reproduce.

I have a 47-204 grille somewhere, but it has been temporarily lost in my recent move. I'll find it eventually, probably when I am looking for something else (which is usually how it goes, at least with me).

Now, after all that verbiage, to answer your question about heat: Heat has been used, with varying success, on various Philco Tenite parts, notably the dials on Mystery Control remotes:
http://www.philcoradio.com/tech/myscon.htm

But it could cause more harm than good. Of course, it's worth a try...what do you have to lose other than a badly warped Tenite part?