I created a strap like this recently where particles fly in from left to right of frame revealing a strap. I created the strap in Illustrator first, imported it into AE and used circular masks to reveal the strap from left to right. By using individual masks, rather than one single mask, the strap looked more as if it was “made of particles”. Then by placing the particle animation (in this case using Particular) above strap layer the whole thing looked nicely integrated. Let me know if you need more details.
JD
John Dickinson
Motionworks
https://www.motionworks.com.au