We really really REALLY want this too. We have a service model where our staff are asked to do work at different locations frequently but we can't do extension mobility for everyone (nor does it make sense when simply working from the office next door for a couple hours as a one-off). Because of this, we have a need for staff to be able to join the respective CSQ where they are working and use their own username and password at the currently seated extension. The only other alternative is for the contact center to be designed around "service accounts" so that the service account is what is used to sign into phones for work - "eeewwww". Service accounts don't allow for accountability and a manager would have to coordinate a schedule to contact center report results to know who what using what service accounts at what time, again, "eeewwww".
UCCX has needed this multiple IPCC extension sign-in ability (maybe based on "phones you can control" in the end-user settings?) for a very long time!
