Yes, the extra damage is maximized on a crit.
From the Players Handbook FAQ:
- Which dice do I maximize when scoring a critical hit?
Only the dice you would normally roll to calculate damage are maximized. If another bonus (like from a weapon or feat) causes you to roll extra damage dice when scoring a critical hit, those dice are rolled as normal.
And from consensus on the WotC forums:
Which dice do you max on a critical hit? If a bonus (like from a weapon or feat) causes you to roll extra damage dice when scoring a critical hit, those dice are not maxed. Other dice like hunter's quarry, sneak attack, warlock's curse, etc. are all maxed. Discussed here and in the 4e PHB FAQ.
I see no reason that a power that triggers on a hit is any different from anything else like a class feature such as Sneak Attack or the free action granted by the Hunter's Quarry power.
As summarized by jprete:
The procedure here is something like the following: figure out what the damage roll would be without a critical hit, considering all effects; maximize that; and then go ahead and add bonus effects that only trigger on a crit, without maximizing them