Tom
I'm not so much as "adding" the uni into a f8 system, rather than replacing the f8 system with the uni and flying the uni without my neck tether that I had been using and instead incorporating part of the anchor bridge into the mix.
I use a sling that has two eye splices. one above the other. I girth hitch the sling onto my floating ring, The top hole is connected directly to the hitch, the splice below it has an extra locking carabiner. I use the extra locking carabiner to bring the system closer to me by ascending up to the extra carabiner and hook it to my ring. There is risk of possibly falling that short distance (2 1/2 ft) but if that did happen i am still tied in. I'll try to take pictures today or tomorrow if it helps elaborate a bit more.
Honestly I think that is the biggest downside to the F8 system, having to safety in and switch the system into a descent mode. I've been playing around with different hitch cord/ rope configurations to minimalize how tight the hitch gets when ascending purely on the hitch. I've been having a great amount of success with 30inch 10mm beeline on velocity, 5 wrap vt knot. I can usually descend a solid 10 feet before it gets too tight. I've also remedied these shorter descents with a reverse footlocking technique (lower my body, lock off on the rope, pull the hitch cord down, continue). It works great for small descents when it's difficult to safety in anywhere. This technique probably isn't the greatest idea for every climber but it works well for me