Depending on the size of the HG, this might be a possibility:
1. Build a HG for every scheme they might use. Actually pretty easy, use the copy/paste function to make multiple copies of the one group and then go into each and drag the members to the order you want.
2. Build a phantom extension.
3. Change whatever currently sends calls to the HG to instead send calls to the phantom.
4. Use REMOTE PROGRAMMING from any phone to manually forward phantom to the desired HG as needed. We'll say phantom is ext 123 and HG for today is 222...you'd dial 359 (remote programming), 123 (phantom), 123# (passcode), 355 (FWD ALL CALLS), 222 (destination). All these strokes could actually be placed under a sta spd dial if desired, may need to use shorter numbers for phantom, hunt and feature codes to get below 16-digit max.
5. If the 5000 is a late version you can also use the CONFIGURATION ASSISTANT to do this by interacting with voice prompts; this can also work from outside the building.