Not a professional opinion, but an opinion just the same.

First of all, I would say to consider what is the most important part
of the item.  For a piano, that certainly wouldn't be the strings,
hammers, felts, etc., as they all wear out regularly and need to be
replaced for it to function properly.  Nor have I ever heard a complaint
that the finish isn't original, unless it's a painted art case.

So then, what is the main part of the piano?  In this case, I would say
that the legs sound like they are at least a very important part, from
your description anyway, and replacing one of the legs would at the
very least decrease the value considerably.

If you take the other extreme and build a set of legs for a piano
that had normal legs to begin with, I would expect that replacing one,
or even all of them wouldn't have that much effect on the value or
classification of the piano.  I wouldn't go so far as to say that you
could replace straight legs with highly carved legs and call it an
original art-case piano.

In this case, however, it sounds as if you don't have much choice but
to replace the leg, and it should be documented with the piano that it
was replaced, even if it's a perfect match.  Still, the piano itself
would be an antique, just a restored antique.  As long as the
modifications are not hidden, I would consider this to be acceptable.

Eliyahu Shahar