Not sure if there's anything special when it comes to root pruning a hibiscus tree, but if its severely root bound and the root ball is full of roots, when you take it out of the pot generally no more than 1/3 of the roots should cut.

Although usually what I do with large plants is depending on how large the pot is and how much roots/how big root ball is, you could probably slice 2-3" off the bottom and slice 1" from each side and the plant should be OK.

Just make sure that any circling roots are cut as well as any tight roots at sides so that the original root ball is broken up a bit and so remaining and new roots will make good contact with any new soil you add.

Thank you for your input.

The best information I found about repotting was at Hidden Valley Hibiscus.

http://www.hiddenvalleyhibiscus.com/care/potting.htm

there is also an instragram video from the hibiscuswhisperer - that shows the process ( i think its still there). generally if severely root bound - cut about a third of the ball off and loosen up the roots, repot in fresh soil. I also like to take a bit off the top when I do this. the plant will be in a bit of shock and frequently drops some foliage anyway. this way you stop that process and the plant works on its roots.