I've been thinking about how to use Unify Standalone for Backing Tracks and I think I may have stumbled across something that appears to trigger embedded instrument layer MidiBox's using Upper Level MIDIBox's from a parent Unify layer (Hopefully the pictures below will make more sense than my crude attempt at explaining this!!!).
1st Unify Layer - This has no sequencing in it. It has an instrument layer set up for "Latch Mode" (and as we drill into it's layer we will reveal the sequencing trickery!!).
The other two instrument layers within the "1st Unify Layer" are nothing special - just a keyboard split for LOWER and UPPER for the live playing
The next two screenshots show you how I use a drum machine to play the same set of drums. Each layer has the same MIDI file (setup to only play one sound) to allow me to have different effects for each drum sound.
All the embedded instrument MIDI files are the same. Except, that one pattern will play for bars 1/2/3 and a different pattern will play for bar 4. This is because the previous layer had the sequencer MIDI files with the long notes.
I would think that you would need to expand the SEQUENCER MIDI files (the ones with the long notes in) to be the length of your composition.
If the SEQUENCER MIDI files are shorter than your compositional backing track, they will just keep looping anyway until you press the Latch Key to stop and restart the backing track.
This technique isn't limited to drum tracks. You could have other tracks for backing melodies. And if you have a MIDI to DMX VSTi it will be possible to control DMX DJ lighting devices so that the lights react with your backing track.
An Update...
If you look at the patches created for the ROLAND CR-78 DRUM MACHINE (in the section for sharing patches in this forum) you will find patches that do a similar job to the above, but instead of using MIDI files for the sequencing, they make use of BlueARP, then the lower level instrument layers have the actual MIDI files using MIDIBOX for each instrument layer with a macro knob for selecting tracks/patterns.