This subject came up again in an operations meeting. As you mentioned, a team's status can be changed to "On Break". If you prefer the Rehab designation, you could individually assign your members to that status by doing the following:
1) In the Configuration tab, in the Mission Types area, in the General Mission Settings section, you could add Rehab as an ICS Position for a particular Mission Type.
2) Back on the Teams tab, set the incoming team's status to Finished Assignment (to make them Available).
3) On the Personnel tab, you can drag and drop individual members into the Rehab box.
4) Later, the members can be dragged out of Rehab and made Available again so they can be put on a team again.