Mapping notes to MIDI keys

[infojunkie]

I'm working on supporting tuning systems with both MusicXML and MIDI. Here's my current understanding:

MusicXML has elements for pitch/alter and accidental which are hardly sufficient on the long term, but are enough for my purposes right now. (For comparison, MEI has a tuning element, which also falls short of generality but is a step further along this direction.)

MIDI supports tuning systems via MTS (Midi Tuning Standard) which allows to retune all or some of its 128 keys to an arbitrary frequency. When retuning MIDI keys, and depending on the actual set of notes used in a score, it can happen that we lose the 1:1 correspondence between the original note name of a given MIDI key and its (intended) name after retuning.

I am stuck on mapping microtuned notes in a score to their equivalent MIDI keys as per a specific MTS setting. I need this to generate the correct MIDI notes from a MusicXML score, where the tuning specification would be given as a parameter (e.g. a Scala scl+kbm combination.)

My preferred route would have been to use a similar approach to midi-instrument and note/instrument / note/unpitched. In this way, I would link the microtuned note to a specific MIDI element that specifies its mapping when it gets translated to MIDI, similar to how a drum note gets mapped.

I don't think this facility exists today in MusicXML. Have I missed something, or does anyone have a different recommendation?