cstPeter: Good afternoon. Check my system #125 and see the same problem I had. The key may be to see what kind of Mylar is used...........
Naturally I'd check with ML to see which type they use....and what they say.
The rest here is my observation:
If you are using a grill and closed glass for a sealed system there shouldn't be much of a problem if it's just for looks and the asthetic and romantic appeal.
HOWEVER: If it's used to heat the room; it's not just the heat, (remember that's radiant waves), when it's dissipated by your rooms small convection currents from fans and vents and windows and such, but it's the particles that will be given off, that may be cause for concern.
(That's why I had to put my Sony in front of the fireplace, {because I was allergic to the particles and other by-products that were given off from different types of logs}, and I could no longer use the fireplace). IF you leave the glass doors open, you will definitely be cleaning your speakers a lot more IMO, and there will be greater variations in room temperatures and humidity values. You may expect room cleaning and a change in air quality, (more filter changes). It seemed more dusty. Naturally, if your not effected, it might not make a difference.
If it were wood logs and a big open system I personally would not recommend it, being they are placed that close.
I would be concerned by the possible change to the paper and the plastic-like rubber seals of the woofer, would they have a tendency to dry out? And would it make a difference if you were to upgrade them years from now for the resale value?
Please continue to check the problem out, as I would also like to find a possible answer or compromise. Best of luck to you and yours.