Has there ever been a stat block for a pre-Medusa humanoid, in previous editions?
Not in 1e, 2e (!), 3.5e, nor 4e core materials.
- 1e MM simply describes medusae as beguiling, shapely, with horrific visages. No mention of how they come to be.
- 2e Monstrous Manual doesn't mention anything of their origins, either. The female medusa entry doesn't even have an "Ecology" section! (We do have mention of the Maedar, male medusa, but nothing about their past, either.)
- 3.5 MM is almost as terse as 1e, but adds that some have embraced criminal activity/formed criminal organizations.
- 4e: nothing about the past.
So 5e is the first time I know of when the MM spells out the origin of a medusa, hewing closely to the classic* myth.
Is there a creature I could base a homebrew pre-Medusa off of?
You don't really need one.
A charismatic, beautiful person is all you need. Or not--see note. The pre-Medusa need not be terribly special in any way, but for whatever will cause her to become a Medusa.
An extant 5e example, with a more fleshed-out origin story. (Possible Princes of the Apocalypse spoilers.)
The Earth prophet is a (male) medusa who "began life as a human aristocrat.... He squandered his family's wealth on endless self-indulgence.... [He] required his guests to don ugly or monstrous masks, thus ensuring that no one's beauty would outshine his own."
"...After searching for years, he found a ring that contained the stolen vitality of a lovely fey creature. As long as he wore the ring, his imperfections vanished and no one could resist his physical charms--until the day the ring transformed [him] into a medusa." (Princes of the Apocalypse, p.9)
* - rather, the revised classic myth. Early Gorgons are hideous, but later they are reinvented as gorgeous, albeit deadly, women.