This is why the LGPL was created, so that you can have modifications done on your library be free-as-in-speech, but still make the library as a whole useable for a wide variety of other projects, including closed-source versions.
Having a separate requirement to email you for a free-as-in-beer license is just overly complicated for this. The more hurdles you put up for people, the fewer that will adapt the library. I think that licensing is one of those cases where is doesn't pay to be clever. Plus, what happens when you decide to stop maintaining the code? Do you want to keep getting emails for licenses years from now?
Edit: in last paragraph, I said free-as-in-speech, but meant beer (see comment below).