I think that an SVG font to TTF converter could
comply with this part of the specification, provided it refused to work on
SVG fonts derived from copyrighted fonts.

that's the issue: refusing to work.

I think that the user should be responsible for his acts and for eventual copyright infringement. There is no general way to know whether he/she has a license granting rights of use of a particular font. Even if the user don't have a license, there are several exceptions for copyright scope of protection. So, there are cases in which even a user without an explicit  authorization from the copyright holder, could effectively have the right to use the work in question. Control should not be embedded on tools because it would never be fair and it would never be accurate.

So, my conclusion is:
I think Inkscape should be able to convert from SVG Fonts to TrueType (or other font formats) and vice-versa without artificial restrictions. Respecting the law is an issue for the user (i.e. human beings), it should not be enforced by code.