I was wondering what you think about using an external tool for compiling the const tables, like Python.
In my opinion, that would be a simple, easy and straight forward solution. And I would go one step further: make the generated files part of the source distribution. That way you don't need Python to compile Fluidsynth. Only if and when those tables need to be regenerated is Python a requirement. And judging from the past years of development, the need to change the tables won't come up very often.
Just think of the const tables as "configuration" and the Python script as a tool to write that configuration. Similar to autotools configure scripts. They get checked in to VCS all the time.
All the best,
Marcus