Can epoxy artistically fill gaps in old wood floors?
In a previous life around 1976 we moved an 80 year old chapel with hardwood floors to a new site.  Between the age, inexpensive initial hardwood, and the move there were many wide cracks in the floor.

The contractor (who built Fort Edmonton) had us mix fine sawdust (from sanding the floor -- so the sawdust was already part varnish) with varnish and stuff the cracks.  Gave it a week to dry.  I don't think the water based varnishes were around yet.  A second coat on the really wide cracks where shrinkage showed, another week.  Then the finish sanding on the floor and several coats of varnish everywhere.  The results are quite attractive, and still look good today.

Given what I've heard about working with epoxy grout, I'd be reluctant to use it without first watching someone do a project with it.  Your situation isn't grout, but the overall purpose is the same, and you have variable width cracks.