Thank you for those thoughts...

FWIW, made an interesting discovery about OBJ+MTL I/O: Material parameters are 'range trapped' at import, but not at export...

eg Load a Poser primitive box from library, dial super-ambient to eg 10. Nice glow, yes ?

Export box as OBJ+MTL, open MTL in eg Notepad. Ka 9.9  9.9  9.9

Re-load this OBJ+MTL, super-ambience is lost as 'invalid' ambience value is trapped.

Re-exporting confirms: Ka 0 0 0

Oops...

Did some digging, found MTL specification should accept values outside 0 ~~ 1 range...