Just an update.
In a fresh full.sysout, I did (FILESLOAD READ-BDF).
Then I constructed a list of the paths to the 17 .bdf files for assembling a composite NotoSans 72 MRR.
(These correspond to the pieces used for the HTMLSTREAM font assembly, in the same order.)
In the XCL exec, changed the *PACKAGE* to BDF and entered
(PROG1 T
(TIME (SETQ NS72 (BUILD-COMPOSITE NSTESTFILES))))
and waited… and waited… and…
Just short of 20 MINUTES later, it finished:

The files total 111MB.

So, now we know that a very large font can be constructed from BDF files.
Matt
(And, of course, I found a couple more bugs that I’ll commit and push.)