Drac, I think your problem is not so much just heat, but the combination of heat and humidity.  You've seen regular posters that weren't dry mounted go all ripply, especially when hung in the presence of a swamp cooler, right?  I think the same thing is happening to your cel, and it's made more noticible by the presence of the spacer between it and the acid-free paper.  

I've had all of my cels, except for the one chroma cel I own (I know, I know, but it's a Magic The Gathering card, so there! :)) professionally framed and I always have a white acid-free mat under the cel.  Because of the nature of the cel, it never lays completely flat, but I don't expect it to.  My husband and I have moved from Albuquerque, NM (very dry) to Milpitas, CA (Bay area and noticibly damper) recently and I haven't seen a large difference in my cels, due in part (I think) to the support from the mat.
My frame sandwich is always mat, cel, mat, mat, mat, UV glass with all of the mats and other materials being acid free.  

The chroma cel also lays directly on acid free foam core and it has not bowed at all.  I don't think you need to worry about damaging your cel, it's probably just fine, just making some adjustments to temperature and humidity! :)