Critr, any quality safely harness should have d-loops on the sides so you can use the safety rope which you normally tie off to the tree trunk as a lineman's belt. Even the basic, no-frills harnesses Summit gives away with its stands have this feature.

All you do is loop the rope through one d-loop on one side, wrap it around the tree, and affix the prussic knot to the other d-loop using a lineman's clip ($5 from summitstands.com, or you could pick one up locally). The prussic knot can be slid up and down the rope, giving you the adjustability you need as the tree's diameter changes as you go up or down.