Well, here's a question for all the 'glider-wallahs, arising from PJ's frustration: would not a 'glider for a 42T ring work with the smaller 40T ring? The fore-and-aft length and hence clearances would work, surely, because of the adjustable arms on the 'glider. The smaller 40T ring would have a slightly smaller diameter, hence a slightly tighter radius than the 42T ring, and that would ensure enough North-South (top-to-bottom) clearance. With the smaller 'ring diameter, the inner edge of the chain (that is, the edge of the chain closest to the ring) could perhaps make contact with the corresponding inner edges of the 'glider -- I don't know enough about the different measurements & clearances inside the 'glider case itself, to know this.
Leaving aside the issue just mentioned, the only possible problem I would see, would lie with the rear portion of the 'glider's chainring assembly -- that is, less than full coverage of the rear portion of the chainring -- and even there, I'm guessing that any exposure of the ring would occur mainly at the bottom & top of the ring, where it exits from the lower cover of the 'glider, and re-renters at the topside. Unless I'm missing something, the entire run of the chain would be covered, with sufficient clearances. (One could even leave out the rear portion of the 'glider's ring assembly.)
I was mulling this over in my mind the other day, thinking about the possibility of reducing my own 38T chainring to 36T at some point in the medium-term future, if I find the hills getting steeper with the passing years. Seems to me that, in principle, a 36T 'ring would fit within a 'glider made for a 38T ring. (Similarly, PJ's 40T ring should fit the 'glider sized for a 42.)
Dan, did you ever try to fit a 38 'glider to your 36T ring? And if so, with what results? I know that Hebie advised you that they don't have a 'glider designed for a 36T ring. Did they ever explain why a smaller ring might not fit a slightly larger 'glider -- say, a 36T ring mating with a "38" 'glider, or a 40T ring with a "42" glider?