Nothing in the pages you referenced supports your claim that damage is rolled before the temp hit points are gained.
Are you sure? Lets read...
All attacks follow the same basic process: (so any attack follow this sequence)
1. Choose the attack you’ll use. Each attack has an
attack type. (ok, you chose your at will with invigorating, you want those THP at all costs)
2. Choose targets for the attack (page 272). Each target
must be within range (page 273). Check whether
you can see and target your enemies (page 273). (ok, that bad kobold near you needs a beating, you target him and he is in range)
3. Make an attack roll (page 273). (ok, you get that lucky D20 of yours and roll a 19!)
4. Compare your attack roll to the target’s defense
(page 274) to determine whether you hit or miss. (Yes!!! Sucess, your attack roll alone is higher then your target defense, you HIT! CONGRATULATIONS!!!)
5. Deal damage and apply other effects (page 276). (Now you deal your damage, THEN you APPLY OTHER EFFECTS, like the +2 to damage rolls for having THP.)
Sorry there Battarager power dosent insert a step called 4.5 that gives you the ability to apply the effects on the order you want it to... But hey, on your action point you have the effect on you before the damage roll! Or maybe if a friendly warlord makes you attack again...