Will the cabin have year round temperature and humidity control now? If not, anything you do will be useless. If so, then you'd have to live with filler between the boards, and color matching it as closely as possible to the wood. Not an attractive look in my book.

No it won't, it will generally be below freezing in winter. Then the woodstove will heat it rapidly a couple days per month. Good point.

In real rustic pieces like what youre describing putting hemp rope between planks will fill the gaps.
This Old House did it in an episode a ways back. Plunge the rope in and stain to match.
They did it in a not so rustic house and it looked ok but i think a cabin setting would look better.

Also since sanding the floor after the rope is installed probably wouldnt work to well so if you do plan to sand id do it before installing the rope.

Otherwise the conventional filler will crack and break out with the seasonal changes.

Definitely do the rope caulk ... it will flex with the floor, keep dog hair out and can be replaced if needed.