On a curiously related note, UVI has a working relationship with MOTU, which in turn maps to MachFive 3, which is the new MOTU synthesizer or whatever one wants to call it, and as best as I can determine UVI IRCAM Solo Instruments and the other UVI sound sample products are compatible with MachFive 3, which is relevant, because MachFive 3 provides scripting capabilities . . .
MachFive 3 (MOTU)IRCAM Solo Instruments (UVI)MOTU and UVI Collaboration (UVI)I have no idea at present whether the new MachFive 3 scripting capabilities work with UVI IRCAM Solo Instruments, but if MachFive 3 works in a way similar to SampleTank (which is the engine for all the IK Multimedia VSTi virtual instruments), then one can create custom instances for specific instruments, which is one way to do something similar to custom rules and all that stuff, except that it is done via a visual interface rather than in a low-level scripting language . . .
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NOTE: The strategy for custom instances of instrumens is that you start with a parent or child preset or sound sample and then modify it, followed by saving it as a user-defined preset, which you then can load as a specific instrument . . . ]
As time allows, I get a bit more information on VST/VSTi and MIDI, but all that stuff is so vastly complex that it borders on being simply mind-boggling, so the strategy I use here in the sound isolation studio is to find specific instruments that do one aspect of whatever I need to do, and then I use the instruments very specifically, where for example if I need a Stratocaster played through a Fender Twin amplifier and a vibrato effect pedal, then I create a staff in NOTION 3 specifically for a Stratocaster played through a Fender Twin amplifier and a vibrato pedal, which I then use when I need notes, chords, or phrases played in this specific articulation, style, tone, and so forth . . .
It is a simplistic way of providing a practical solution, and it tends to require a virtual festival of cloned NOTION 3 projects due to the current 32-bit application workspace limitation of approximately 25 "heavy" VSTI virtual instruments per NOTION 3 score (a.k.a., "project"), but I have a system for doing this, and it does not take a lot of time to do stuff this way . . .
Another advantage of creating songs this way, which specifically is focused on doing instrumentation in
layers, is that what initially appear to be very simple notes, chords, and phrases interact to create elaborate notes, chords, and phrases that are
not present in the original notes, chords, and phrases of the individual instruments, which is a
Gestalt phenomenon, where the whole is more than the sum of its parts, which is fabulous . . .
Fabulous! 
P. S. As part of the research for a reply to a different topic in this FORUM, I called MOTU Technical Support and verified that MachFive 3 works as a VST plug-in, which maps to MachFive 3 working with NOTION 3, and the new scripted instruments in MachFive 3 also work this way, since everything is done via the various MachFive 3 interfaces, including when MachFive 3 is functioning as a VST plug-in . . .