Tex: I have only 3 rooms left with the original birch floors (half to 2/3 of the main floor), basically three 9x10ish rooms. I have NO closets, and NO other wood scraps that I could use to fix or patch bad spots. I have *maybe* one or two square feet that was left under the bottom step of the stairs that I could pull-up to patch holes in the floor, but none to replace anything extensive. This is why I'm reluctant to pull the floor and re-lay it.
I did check the products listed above, and I'm not confident that either would work well for my floors. The trowel grade stuff is "more liquidy" wood filler (which, as a cabinetmaker, I'm very familiar with, and I'd NEVER use for floor gaps), and the other one is regular water based crack filler (same thing), and the Silkaflex is just a construction adhesive, which isn't really meant for crack filling, and would be a mess/nightmare to try to put down. Not to mention that it's the type of glue that probably costs 8-10$ per tube, and I'd need like 20 or 30 of them.
I still need to read-up a bit more about the epoxy type filler. The problem with this one will likely be that it's not very flexible.
I think I read on one site that someone used a mixture of sawdust and varnish to fill their gaps (made into a putty). I'm also wondering if the "glue and sawdust" woodworker's trick would work or not. PVA glue is still flexible after it fully cures. This fact is evident by the drips and blobs that I regularly scrape off the glue bottles at work - most of which (blobs) can still be stretched quite a bit before they tear.
As a side note, and as one of the reasons that I want to try to find a filler, is that my dad filled some cracks in his floors when he redid them (likely in the 70s or early 80s) and the putty did crack slightly, but it never popped out. He did this in only a few places though, but the floors still look amazing over 30 years later. I think he used just regular wood filler.