Having done much research about "font tweaking", I now came across this highly interesting similar "music tweaking" case, namely the music performance forgeries by Joyce Hatto, described in much detail here
and in dozens of other documents linked from this Wikipedia entry.
For instance, we read that the "music tweaking" (forging) of Chopin Mazurka performances was achieved thus:
"The CACD20012 release has added filtering, and the speeds of each performance pair vary slightly in the range of a few percent (+1.2%, -2.8%, and -0.7% for three sample mazurkas)."
This method of "music tweaking" (music forgery) is very similar to the method of "font tweaking" (Fred Smeijers).
At site http://www.creativepro.com/story/feature/21883.html we read:
"Smeijers introduces the term "font-tweaking," which he clearly doesn't think very much of, to describe the currently common process of simply taking an existing typeface and ringing a few obvious changes on it, then turning around and presenting it as a new design. Some of the results are simple rip-offs; others fall into a wide gray area - where does influence give way to plagiarism?"
see http://ospublish.constantvzw.org/?p=183 and similar sites.