I'm just about to setup a HTPC with a HVR-2200 and Media Portal, but on Vista - so I hope your solution works for me as well!
To make sure the fix is fully working, I was wondering if you had tried the following tests:
(1) Start recording a digital channel; then see if you can switch between watching analogue and digital channels (on a different mux to the one you are recording)
(2) Start recording an analogue channel; then see if you can switch between watching analogue (different to the one you're recording) and digital channels
I'm thinking that the scenarios you have tested already don't force a tuner to go from being analogue to digital, and vice versa.
Would be interested to know what you think / find out.
I did a similar test. I disabled some of the tuners so that i was only using one hybrid tuner. I was able to switch back and forth between analog and digital using the one tuner. This is in XP Pro SP2 / MP RC2 / HVR2200. Hope that helps you a little.