I think a makeshift deorbit burn calculator could be added to SSU Workbench, by (somehow) giving it the vehicle state vector and a target runway, and it would calculate if the deorbit burn was done now, targeting a certain FPA at EI, what would the range and crossrange be for the target runway. If those parameters are within the limits = deorbit opportunity, if not, propagate the state vector 1 minute (or something else) into the future and try it again, etc, etc...
Yes, it could be geologically slow to search a couple of orbits, but as it would not run inside Orbiter it doesn't hurt the sim performance.