I myself had written a tool like yours for a similar purpose except I needed it done quick'n'dirty, so I wrote it in C with GTK and just used a grid of toggle buttons :D

It all went fine until I needed a variable width modern-looking font, so I wrote a convertor from BDF to the proprietory format the video chip used (vertical middle-endian scan lines arranged in a weird order) and generated the BDF using gbdfed or FontForge, I don't remember which one. I must admit the convertor was horrible :D

Interestingly enough, when I needed fonts/sprites again in a couple of years, I resorted to a set of multi-layer GIMP files with a set of rulers and grids which I would export into the X bitmap format (which is a text format consisting of a bunch of C constants). (I didn't need to dynamically generate text on that project, just fixed pre-prepared strings.)