For what it's worth, I allow modifications of my fonts because I am only secondarily a type designer and primarily a graphic designer. The number of times I have needed to modify a font in the last ten years is under a dozen, but each time the change made a huge difference to a job or even recurring tasks. Reasons for needing a mod have included an errant kerning pair, a missing character (é - I'm looking at you) and problems with glyph outlines that made the postscript printer puke spikes through my text. None of these were hard things to fix, and the production environment I was living in would not have allowed time for me to politely inquire of the original designer to fix them. The idea that I shouldn't be allowed to fix errors is absurd to me, especially when I'm dealing with a font that comes from a lessor designer (in which category I include myself). The idea of a no change clause strikes me as more specific but in the same category as not allowing fonts to be sent to a printer. Impractical.
A clause that states that the designer is in no way responsible for modified fonts is eminently reasonable.
I immediately thought of those modular type ornament blocks. I can only find one example right now. Sorry for the bad quality, my scanner is not working, but this is from The Lovell & Gibson Specimen book of 1847 (in reprint); if you look close you can see the individual blocks that it is built from.