OK, my take on it, is to get the interior up to the highest rH it will see in the humid season(NOW), for a week or longer. Then fill the gaps. This allows the wood to swell to its largest dimensions, and won't squeeze the filler out, as it will, filling during the winter-heating-dryingout, months and come the humid months and the floor swells. During the winter, the floor will shrink, unless you have a humidifier. You will see gaps appear.

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.

Jane its a Contractor thing I am thinking. For my part I hae been out on service calls from the Wallet that bought a house where the last owner had done something like that. They are plenty mad.I mean really reallly mad. It would be better if you just put a coat of finish on that deck, that is better for your customer, of course if you dont care and let the buyer beware is the goal you probley could get away with latex caulk and heavey bodied paint. Then in a very short time all that caulk is pushed up taking the paint with it the thing looks like hammered poo poo (see how good im being ?? ) and someone like me gets a call to fix it. I give them a bid,get yelled at and scooted off the yard. Then when this new owner goes to sell something he remembers this, pulls the same thing on one of his buyers. Worst case so far>> 200 ft of lamanated curved benches around a pool, the whole look is centered/picture framed around these benches they also acted as a rail against the 8' drop behind them. They are also completley rotted out. HO hires painter who glues them back together with latex and white paint. Bang Bang I get a call because the new lady owner sat on them one time, $12,000 dollars later they look steller but this lady now caries a glock just hopping for a lucky shot.

design feature. an old furnace register was removed from the corner of our library. the contractor wanted to weave in boards to make it go away. i said no way, had them just fill in the original frame with new maple. underneath the carpets in the dining room and library are wide pine boards... the original owner couldnt afford the maple everywhere, so it was just done as a border and the center was covered with rugs or lino. i'm sorry they don't show! just because its partially under your table doesnt change my answer. i love seeing bits of the past in our every day lives. oh, and i want to see pics when you are finished!