I have seen this used to good effect also. It never made sense to me, but it worked. BUT I think this is only possible with extremely small, cosmetic cracks that are not likely to open and close. When you have wide boards you will almost always have some opening and closing of the seams and usually filling these voids is not a good idea.
If the plank floor gaps are 1/4" wide or smaller then I wouldn't bother doing anything. If you MUST find a filler then the only thing I would consider is this 3M stuff that is mad for filling gaps between teak decking on boats: http://www1.3m.com/marine/images/Marine_1999_3.pdf See page 100 for the stuff and for some rough instructions. This stuff is flexible to allow shrinking and swelling of joints, yet it is sandable, etc. I think you will be stuck with black color though... If your gaps are a full 3/4" deep then I would use some sort of hemp or foam backer filler for about half of the depth, otherwise you will use too much of this expensive goo and also it won't be able to compress enough if it is that thick, assuming that the boards will swell in the summer. I wouldn't do this project in the height of the summer or the winter b/c the boards might either be too swollen or too shrunken. I would do it right about now, where the gaps are probably about middle sized for the year. Good luck!