I would say he has to do it in one round, after all he isnt harming the same spot the next round unless he has a way to stay in the same spot. i think the grapple check would be a good system to use. Each round traveling x feet unles a grapple check is made.
If I remember correctly, the problem was the original Draeden had tubes that were literally miles long (8.6 miles for the
smallest of Draeden). At 60 feet per round it would take 757 rounds to reach the stomach - an hour and a quarter!
That's so long most encounters would be over before the swallowed victim is a hundredth of the distance.
Furthermore, it means a larger Draeden would take longer to swallow an opponent and do full damage than a smaller one.
I suppose you could set the "Swallow Speed" to an arbitrarily high value, like X thousand feet per round, so they reach the stomach in HD/X rounds. Alternatively, set the speed to one-tenth or one-fifth of the tube length, so the victim always reaches the stomach in 5 or 10 rounds.
It's just easier if the swallowed victim reaches the gizzard the following round, as per the standard Swallow Whole rules.
Obviously this means the swallowed creature would have to travel very very fast (at least 5164 miles per hour according to my calculations, or roughly Mach 6.8!). Maybe some sort of space-warp process is involved, or a variation on the notorious Ready Action Linear Acceleration...
