Yes for BFTA Shoot Rules but you must remove the sling from the arm and it MUST remain attached to the rifle for the entire course. Good luck with the dangly weight flopping about and of course the end cannot touch the ground for sitters.
Basically, no item can be attached or detached once the course of fire commences.
And, yes, I think it is a pain and have such sling and jacket sleeve hooks with easy detach handstops for 25m indoor .22RF but there hasn't been any budging.
That is for UK events, I don't know how other countries rule it.
BFTA Secretary 2012-2014
CSFTA Secretary 2014-2016