More details-there is panel molding on the wall in a lobby. There is also
a section of wall that doesn't have the panel molding on it, and the
owners want to add some. It doesn't have to be the same, since I'll never
be able to find a match, but should be similar. Googling led me to
suppliers (one in Ireland!, when I spelled it "mouldilng"), and some might
be ok, but I'm looking still for a closer match.

Do you think it would be possible to make a latex mold of the (E) wall
molding, on the wall, and use that to cast new pieces?


Yes.
Build a trough, plug the ends with foam, fill it with latex-moldmaking compound,
and brace the whole thing against the wall until it sets. (does latex need a
parting
powder to keep it from sticking to/staining the wall? It probably depends on how
porous the plaster is.)

Another option if you've got decent woodworkers and toolmaker available to
you is to take a profile using whatever that tool is that looks sort of like a
comb with touching teeth, and use that to make either a router bit or a
blade for a molding plane. Since you appear to be talking about curved
sections of molding, the router bit would probably be easier.