I guess I misunderstood. Is there some other hard baked value elsewhere? I manually edited the underlying mfcc-gui.maxpat to have a higher maxnumcoeff but I still get the zeros:
As in, the fluid.bufmfcc~ object is now this:
fluid.bufmfcc~ 40 @features #0tmp.mfccs @blocking 0 @numchans 1 @minfreq 200 @maxfreq 10000