Funny this thread appeared when it did...only days after I'd completed a sketch of the original "Slavers" series pre-gen characters.
"Elwita" is her name, btw...the female dwarf from that series who is plastered (in beautiful Jeff Dee glory) on the cover of the first module.
I, myself, never had anyone want to play a female dwarf, so it was not something I ever put much thought into. But, having now thought on it, I think I would make it a matter of player perogative. Some clans' females grow beards, some don't. Previously, I always drew/thought of female dwarves with the bushy sideburns kind of thing but not a full beard. But, ya know...why not?! As a matter of cultural taste for beauty, no reason some dwarf females couldn't have beards.
I recall, from the Basic rules red box (I think, maybe the original Expert set/blue box) the description of dwarves was rather clear..."Dwarf males wear long beards. Dwarf females wear short beards." or something like that.
I'll scan the image in here if/when I have a chance. I really like how it came out.
Anywho, to the original poster's point, when I was doing the drawing, I did find myself realizing that this module was the only place I could think of (or had in my possession) that had illustrated a female dwarf character...In/across all of the breadth of original BECMI or AD&D (1e for sure, possibly 2e too), dwarf females were simply not illustrated. It was just kinda assumed there were some somewhere.
Glad to hear there's more out there now...but I do have to say, out of reverence for the "classic" module, I did do Elwita, as shown, with beard.
Have fun and happy shaving.

--Steel Dragons