It is not possible to watch copy once protected content in XBMC, and it probably won't ever be. The only way to watch copy once protected content is Windows Media Center. You can use multiple HTPCs with the HDHomerun Prime, just keep in mind that if you record something on one HTPC, it won't be watchable on another HTPC (again, just talking about copy once content). Also, recording schedules are not shared across multiple WMC machines. So say for instance you like watching Breaking Bad and wanted to be able to watch it on any TV in the house, you would need to set it to record on every single HTPC.
That whole setup is extremely messy IMO, and not worth the effort. A much better option is to use one HTPC, and then use WMC extenders like the Xbox 360 for all the other TVs. WMC extenders are completely immune from all copy once protection, and changes to the recording schedule are reflected on the HTPC and all of its extenders. You also only need to worry about having hard drive space for your DVR at one location (the HTPC).
Unfortunately extenders, while perfect for live TV and DVR, have limitations when it comes to personal media. For instance, the Xbox 360 can't handle a 20+ mb/s blu-ray rip.