From what you have described, plaster and lathe, I wouldn't think the
plaster and lathe is along the bottom of the stair stringers, but
encases some cavity under the stairs. Since you will open that area,
why not finish the interior, there, for storage or other and create an
access door, concealed, maybe, if an obvious door would not complement
the surounding design/decor/trim.

If there is such an area, accessing a tread should not be an issue, in
order to secure the tread easily.

Sonny