Figured out a less than perfect way to do this:
1. Create a new folder on the IIS server and save the DLL to it. You cannot publish server controls despite the fact they are installed on the server. Go figure.
2. In the consuming project, select Add Reference (not a .COM or Web reference - just Add Reference!)
3. Select the Browse tab, browse to the shared folder where the DLL was saved, select the DLL, and click OK.
4. Then you also have to add the control to the toolbar by right clicking it, selecting "Choose Tool" and browsing to the shared folder where the Control resides yet again.
Then you can drag the shared control from the toolbar to the page.
It also appears to be necessary to remove and re-add the reference to every project using it whenever the control changes, so be sure it works correctly before adding it to umpteen different projects!