Well, you can put VERSION="..." in a hintfile, does this do what you need?
(it is a horrible hack, sometimes it will fail, but usually it will work perfectly
SBo's qtkeychain is 0.1.0
The latest qtkeychain is 0.3.0
=> put this in qtkeychain.hint, it will guess the download URL with sed
SBo's lyx package is 2.0.4
The latest lyx is 2.1.1, but the download URL can't be guessed automatically
=> put this in lyx.hint
In both examples, if you know the new md5sum then you can add MD5SUM=".....", or else it will ignore the md5sum.