I don't know enough about install.rdf files to know how to tweak
TiddlySnips but is it possible to add support for SeaMonkey? I think
maybe all that is needed is the addition of a
<em:targetApplication>...</em:targetApplication> node with the correct
<em:id>...</em:id>, <em:minVersion>...</em:minVersion>, and
<em:maxVersion>...</em:maxVersion> values (possibly
<em:maxVersion>2.*</em:maxVersion> or <em:maxVersion>2.1</
em:maxVersion>). Maybe something like
http://ted.mielczarek.org/code/mozilla/ffinstall/
or
https://addons.mozilla.org/en-US/developers/tools/builder could be
used to create it.