Use the smallest hoop possible and be careful that you don't stretch the fabric too much.
Also, if these wristbands are terrycloth, you may need to use a topper such as AquaFilm or Solvy (looks like plastic wrap you cover leftover food with) so the stitches don't sink down into the fabric.
I have embroidered numbers on wristbands before. Definitely use topping. I used an 8 cm hoop. Had to be careful about keeping the underside of the wristband from getting caught up in the hook. Pretty much had to hold the underside on each one. It can be done, you just need a small enough hoop and topping.
We use the fast frames and have no problems with wristbands. We do not have the SWF machine though. I think the fast frames work on a lot of different machines though. You may look at fastframes.com.
its not that hard to embroider a number on a wristband just use proper materials. and follow procedures or you can search the internet on how to embroider a number on a wristbands.
I have a SWF 1501T and I use my fast frames for wristbands, socks, beanies and washcloths..
I use the lightweight sticky back from Gunold and solvy on the top.. works great.
I don't recommend it if you have a great number to do but you could try the longer hoop line them up and clip them with some clips to hold them on..
Embroidering small items? 
