The obvious thing to me would seem to be to store the current versions in debian/source/versions
This file would consist of lines of the form URL space version-number new-line.Although human readable and part of the packaging source code, it would be maintained entirely by uscan and driven by the watch file. uscan would not bother to create a versions file if there was only one rule in the watch file and the version number in debian/changelog made sense to it.
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