Way back when this was the "Groove Matrix" in Project5, I experimented with using MIDI Learn, one-shots, and triggers to create Matrix "instruments". It's loosely along the lines of Foley effect triggering, but with individual instrument samples mapped across the entire MIDI keyboard. The P5 iteration was fairly buggy when you pushed it, so I abandoned the explorations. I haven't used the Matrix since in Sonar.
But - as one example - I would load up some of the individual Dimension Pro samples (conveniently labeled with the MIDI note number), and map with MIDI Learn to the appropriate (or inappropriate) keyboard note.
Don't forget that MIDI CC and "positive" Pitch Bend are fair game for trigger control, too. Mix & match for guitar, piano, synth sounds, FX, etc., or stay conventional with one instrument's sample set.
There are some unusual, cool gated & overlapping sustained effects to be had with various trigger and loop settings. "Quantizing" to a particular scale or remapping the keys are real possibilities. Thanks for the discussion, Craig. I may just have to revisit this Matrixsynth.