Might play a game tomorrow with Dash.
Have I understood it correctly that the official answer is (for competitive play) :
1. Declare barrel roll to one side.
2. Before measuring declare if you are using ability.
3. If you are NOT using ability and it DOESN'T work you cannot declare same side again.
I believe this is correct, I do not see any other way to read the rules than Shado's reasoning here.
When declaring you are not using Dash' ability, Dash would be treated as per normal rules when boosting/rolling.
As per competitive play wording regarding boosting/rolling onto an object:
Barrel Roll - "...the player may declare a barrel in the other direction, or he may declare a different action."
Boost - "...the player may declare a different maneuver template or direction to use, or he may declare a different action."
A boost or barrel roll action is still that action, whether Dash' ability is in effect or not, so if you cannot boost/roll in the direction you first declared, as per competitive play rules regarding declaring direction, then you must either go to a different, legal area, or choose a different action altogether.