According to the which I sent to the mailing list, I believe it would be nice if Fossil would extend its current support for import/export from/to Git by adding support for incremental import/export according to the Git's fast-import/export docs:

<verbatim>
 --export-marks=<file>

Dumps the internal marks table to <file> when
complete. Marks are written one per line as :markid SHA-1. Frontends
can use this file to validate imports after they have been completed,
or to save the marks table across incremental runs. As <file> is only
opened and truncated at checkpoint (or completion) the same path can
also be safely given to --import-marks.

--import-marks=<file>

Before processing any input, load the marks specified in <file>. The
input file must exist, must be readable, and must use the same format
as produced by --export-marks.
</verbatim>

This would (hopefully) enable Fossil users to keep using Fossil while collaborating with other SCM systems, something what one can e.g. achieve with Bazaar by using bzr-{hg,git,svn} plugins.