I filed a GM petition on the subject the first day of 3.3 release about this issue. It took them until last night to answer it.

The answer was as simple as this:

After each random heroic instance, disband your premade group, reassemble using a different member as the group leader. Queue for another random heroic. If you get a duplicate heroic that you've already run with the group, it will be reset and bypass the instance locks.

I tried the GM's solution.

It does not work.

Any instance I've done with my box team earlier in the day is still locked, regardless of disbanding and re-assembling the team with a different leader.

I have two tanks, so what I did was replace my primary tank with my secondary tank, leaving the other four members the same. I queued for random heroic with the new tank as lead. First heroic was a duplicate I had already run with the four and my primary tank, Gundrak. The instance was indeed reset and locks bypassed.

Basically it seems that if ALL members of the team have done a given heroic, then the heroic will not be reset by the random heroic instance finder. Likewise, it seems that if AT LEAST ONE team member has NOT done the heroic, then the heroic will be reset.