You can do that now.
The Playable RealTracks should be working on Drums but they haven't got them enabled yet.
If you load Sforzando into a Utility track and choose a PG Drum Kit,
you can then select Piano Roll, select a Utility track, drag n drop a midi drum solo into it setting the bars in to position the midi.
Go into the Audio Edit for the RealDrum track and mute those bars that have midi. Adjust the volume of the midi drums to match the RealDrums.

Video: BB22-Drum-Solo.mp4

MidiSolo to try DrumSolo.mid