I've done it to see how it would work. It's not conceptually hard, but
as you can imagine, the changes are pretty widespread, affecting both
Tcl and C code in something like a dozen files. That means that maintaining
your changes across Tkined releases would be pretty painful, though Juergen
has said that he has frozen Tkined development until he can put time into
a major rewrite.
In any case, if it meets your needs, it would be a lot less work to try
Buz Owen's suggestion and just add some attributes. That can be done
entirely using Tcl code.