: . . . Is there any reason why I can't mix sanding saw
: dust with the epoxy I am using to fill the holes.

This should work fine. If you are working with mostly cedar, you may want to sand a piece of scrap pine just to generate some lighter dust to mix in. Sawdust/resin mixtures tend to dry a bit darker than resin on solid wood.

You can always mix small quantities -- just a few drops at a time -- of resin and sawdust to try on a few holes. When you get one that looks good mix up slightly larger batches to complete the rest of the filling job.

Hope this helps

PGJ