Oil-fired furnace sight window repair
There certainly are high-temperature epoxies that should do the trick for this (J-B Weld comes to mind,  it's rated for a constant 500 deg. F but you should confirm that's higher than this part of your furnace gets).

But you say "my assumption is that the metal groove for the e-clip has degraded" -- does that mean you're not sure?  I ask because to me this seems like an odd thing to wear out given that it's just holding a stationary piece of glass in place.  Is it possible the clip popped out a long time ago and the groove is just full of soot or rust?