The WFTF rules have undergone quite alot of changes recently, and one of those is to do with attachments to rifles.
Writing new rules for the WFTF has been a difficult task for the core WFTF Committee and the RGB's but to help with difficult decisions we try to keep in mind the core spirit of FT. And that is essentially that the rules need to have scope within them for innovation so the sport doesn't stagnate and also it's a part of FT history that people can invent wild and whacky ideas to improve performance. The flip side of that is that instead of hard and fast rules defining everything we try to write rules that could better be described as 'boundaries' than rules.
Also inbuilt into those boundaries is a balance of advantage and disadvantage. By that what I meanis that if someone develops something that clearly gives them an advantage, there should also be a disadvantage attached to give balance.
So onto your sling. The use of a sling gives you an advantage, and so the disadvantage that's been applied to the use of a sling is that you cannot attach and detach a sling from your gun. If you have a sling attached, it stays attached for every shot. The other disadvantage applied is that setting up your sling is deemed as part of setting up time, so it's to be done while the stopwatch is counting down. You get 3 minutes for a lane, that's 1 minute setup time and 1 minute per target.
In short, no your sling isn't legal in the way you intend to use it. To make it legal you'd need to leave the entire sling attached to your rifle throughout the competition. You can strap it round your bicep, but you must do that after you've started the stopwatch countdown.