I think that a lot of the larger extensions weigh more than six ounces so they are likely to put a cue over the limit. I would rule that if you use it rarely to reach otherwise impossible to get to shots, it's OK but you should not be using it for normal shots.
The current cue specs are, in part:
17. CUE STICKS
Cue Sticks used at WPA competitions should comply with the following
during play at table:
Length of Cue: 40 inches [1.016 m] minimum / No Maximum
Weight of Cue: No minimum / 25 oz. [708.75 gm] maximum
Width of Tip: No minimum / 14mm maximum
I think there are some problems with these specs. I would prefer to see a minimum weight maybe larger than six ounces. Clearly a 1-ounce cue is not part of normal play.
Similarly, I'm not sure why there's a 25-ounce maximum since I don't see how going to even 40 is going to significantly change how the cue hits the ball compared to 20 ounces.
Also, some masse cues are larger than 14mm. I have an old book that actual recommends 16mm as standard for normal play. I think 25% larger than 13mm would not change the game insignificantly.
Consider this cue which would be permitted under current equipment specifications: a one-ounce balsa cue with a 6mm tip and 41 inches long. Perfect for delicate safeties.