From a letter of the law, there is no specific prohibition.
From a spirit of the law, the "straight line" restrcition should prohibit a tumble since you really cannot easily justify s stright line and tumble to avoid attacks.
However, rather than just ban this combo, allowing for nods to erroll flynn style charges, forward roll and strike as roll commences upward type maneuvers we have all seen in movies...
Allow it but apply a serious penalty to the tumble check. maybe -5 or even higher depending on how much you think it inhibits. -10? At -10, assuming a 20 dex and MAXED tumble, a 16th leve rogue with tumble+19 could automatically charge-tumble thru reach hexes to strike an ogre/giant. That doesn't sounds totally out of whack. At 6th level, he can auto-tumble thru without the charge.
All in all, it looks like a great representation of high skill level use.
So, IMO, make it harder, not impossible. As a general rule, my first choice is to go with -5 to -20 penalties for those actions that my gut says "nah" to. Let them be impressive.