Hello Werner and all,
I’m among those developers “craving” for a tool to create test fonts, specifically to write unit tests for fontTools.
I checkout out the 010 Editor and the “TTFTemplate.bt" you linked to. It doesn’t look much different from what the sstruct.py and sfnt.py modules already do within fontTools. For each sfnt table, fontTools provides a template for (un)packing binary data (to)from predefined types.
I guess the reason why you believe the test font generator “must be on a far lower level than ttx”, is that the latter is meant to produce “valid” fonts, so it doesn’t expose all the intricacies of the sfnt format which we would like to test (this incidentally is what makes it a “textual, human-readable representation of a font”…).
I’ll try to see if I can come up with some solution built on top of fontTools, and let you know how it goes. Of course, any suggestions would be much welcome.