tab rhythm signs are below staff

Fair enough.

Your statement "Using colors to indicate the different voices of the polyphony in historical lute tablatures simply does not exist" is contradicted by historical 16th-century Spanish vihuela tablatures (Fuenllana, Pisador, etc.) This method appeared in historical prints to indicate which voice of the polyphony was meant to be sung as the player struggled through a five- or six-part motet. Of course these niceties disappeared in the 20th-century black and white facsimiles, but can be found in online sources scanned in color.

Yes.
If you play it back then you can hear that it is correct. If you inspect the piano roll editor you can see that it is correct. The only problem, (a very real one), is that the notation reader cannot readily see that it is correct because the stems cannot be distinguished per voice.

My understanding is that the numeric/fretwise underpinnings of guitar and lute tablature are the same. But to display French lute tablature an encoding system replaces the chars 0 through 9 with glyphs a through j. (Note that Italian tablature uses numbers like modern tablature.)

So for French tablature:

0 displays a
1 displays b
2 displays r    (r because a con a tablaure line may look like an e)
and so on:
3 = d
4 = e
5 = f
6 = g
7 = h
8 = i
9 = j

I thought the encoding might exist in the font itself, by having:

    the lute a glyph in the Unicode UTF-8 encoding slot U+0030 (for the char 0)
    the lute b glyph in the Unicode UTF-8 encoding slot U+0031 (for the char 1)
    etc.

But that wouldn’t account for fret numbers higher than 9 … and when I set the fontFace for lute tab to MuseScore Tab Sans, the letters a through j persist for numbers 0 through 9 (UTF-8 U+0030 through U+0039. So museScore must have an internal mechanism that handles the situation, and allow k for 10, ** l** for 11, m for 12

… unless the lute font has a 10 glyph at U+003A, an 11 glyph at U+003, etc.

Anybody know how this is managed?

More on Unicode font encoding.