This would give a good way to know when the charcoal is ready and i would say glass should be able to withstand the temperatures posed by charcoal making, since bricks also do.
Meta Info

• TFC Version: 1.0.12.141(at time of request)
• Is this a compatibility feature for other mods? Or a enhancement for TFC alone?
• If necessary, what other mods (including versions) would this require?

Have you actually tried this, and found that it doesn't work?

yes