(A long-time fossil contributor here.)
Fossil has no such feature. Fossil sync synchronizes the remote and local saved state (checked-in/saved state only). It does not do anything with un-checked-in state.
That said, you can use fossil's stash to periodically take a (non-sync'd) snapshot using something like 'fossil stash snapshot -m "snapshot @ $(date)"' (i have that aliased and use it often). That does not enter the SCM history and is lost if you destroy the checkout dir.
‘sgbeal and I were doing some fossil dev work ourselves (I’m personally not at his level of fossil-fu, but am a long-running user and contributor). Our work was in a fossil repo (he in Europe, me in North America) and we were using the chat[0] feature to discuss our work when we noticed and discussed the GP post. Fossil has been self-hosting for ages, now is it self-correcting? /s