Hi there!
I don't know if I understood you properly. But here we go.
Since you can create the bitmap font in any image editor, I think it is kinda useless to create a parameter like that. Because, if you are creating the font, you can already create it in a "monospace" way. You can even choose the size in pixels of the blank space by the spacebar and also the space in pixels between each character:
Look at this example, with this settings.
Setting A - The space between characters is 0 pixels and the blank space(using the spacebar) is 4 pixels. Here is the results.
Setting B - The space between characters is 4 pixels. The blank space(using the spacebar) is 8 pixels. Here are the results.
Are those kinds of effects you are looking for? Or maybe I misunderstood the concept of monospace font?