I've done it both ways...filled and unfilled. I like using black epoxy to fill cracks, but I usually don't fill large voids, as I think it looks weird. On that piece, I'd probably leave it as-is.
On a related note, just because a piece of wood has a bark inclusion or natural void doesn't guarantee it'll look good. I've made a lot of pieces with natural "features" that look like they're missing something. Look at the piece from an aesthetic standpoint, not just from a "this is challenging" point of view. Here's an example of one that didn't look as good as I thought it would:
Vaughn I like the way the tables look with a bright colored epoxy. Kind of leaning towards a brilliant blue or magenta. I do see what you mean. I've trashed a few if I couldn't find or foresee a good option.
Love the look Vaughn has on his bowls with big voids. For a hole the size and shape of yours, Dave, I would pierce it on each side and lace it up with a leather thong like shoe laces. Gives it a really neat look..
With the shape of what you have going there, I'd leave it open. Nice rustic look to it that way. Sometimes color is good, sometimes it's bad. I have found that my customers prefer no color, but as natural as you can get away with while turning it.
Another vote for leaving it open.... I've only done 2 or 3 bowls that had openings such as yours, but they seemed to have been the first to sell... people seem to really like the artistry of bowls like yours.
First one I ever did was an ugly piece of work... it was a chunk of oak that because of all the holes, I didn't dare try to get the walls thin.... I showed it at a craft show and a fellow looked at it, put it back and walked away... about 20 minutes later he was back and grabbed it up and said he couldn't just walk away, he had to have it... to me it was ugly