Hi Torben,
I do this all the time. To add midi to a registration is easy. Load your midi into Song A. Then create your registration. When you memorize the registration button, make sure MIDI Song is checked off. This will tie the MIDI to the registration. To only play only the solo all channels of the MIDI can be turned off except the channel the solo is on by pressing the Assignable button B, unless you changed the default setting for this button. Now if you look on page 66 of the reference manual you will see a picture of the Song A window. If you look to the left of the play pause button you will see an arrow pointing down with three lines above it. Click on this and save your registration memory again. This only has to be done on the #1 registration button. What this does is when you have sync start selected for the rhythm to start when you start playing, this will start the midi at the same time. Now the hard part is getting the rhythm of your registration to match the speed of the midi. An easier way is to use the rhythm and backing tracks of the midi. Turn off the lead parts in the midi and use your registrations for your left and right hand to play along with the midi.
It sounds a lot harder to do than it is.
Good Luck!
Happy New Year!
Rick D.