I have an 18th level half-orc barbarian, with four arms, beast-totem claws, Improved Grapple, Improved Savage Grapple, and Power Attack. It looks like the party is likely to encounter a pseudo-beholder (home brewed by the DM, 15' across and heavily armored) and I'm considering whether diving into its mouth is a viable strategy. While my barbarian would have the grappled condition, he would take no Dex penalties, get full attack bonuses, and full combat maneuver stats due to the Improved Savage Grapple.
He would get four 1d8 claw attacks, at +21 to hit and +20 damage each round. Meanwhile, he may take some bash damage, but I'm pretty sure he'd give much better than he got.
Thoughts, flaws, etc.?
Unless it's actively trying to eat you then no.
I get the impression you are trying to give yourself a mechanical advantage, you can't really decide to jump down a creatures throat anymore than you can say you can claim to drive a sword down it's throat. You just need to be beat it with the mechanics given.
and if your DM did decide to use the Swallow Whole the damage you will be doing is pitiful, a creature only takes a set amount of damage if any at all before you are ejected from the wound you inflict.
If you end up being the one that kills it then you are perfectly free to say that you did so by diving into it's mouth and ripping it up from the insides.
You can only do a limited amount of damage to the insides before you "escape" or fall out of the hole you make so it's probably not super viable