I've made a discovery with how noise instruments work in R/G/B/Y. That "unknown variable y" that I mentioned is actually part of the latter variable "z." To correct myself, I'll now classify the variable that determines the "type" of drum strike as "yy." These are what the different values for "yy" gave me (mind my lack of proper terms):
00: Nothing
01: Deep snare hit, long
02: Deep snare hit, medium (ex. Route 24)
03: Deep snare hit, short
04: Deep snare hit, low velocity
05: Five-note drum rise (ex. Title Screen)
06: Low-pitch 256-byte sound (ex. Vermilion City)
07: Low-pitch 256-byte "double" sound (ex. Viridian City)
08: Soft snare hit, short; decay
09: Mid-low snare hit
0A: Mid-high snare hit
0B: Deep snare hit, extra long
0C: Soft snare hit, medium; decay
0D: Soft snare hit, long; decay
0E: High snare hit
0F: 256-byte sound-to-deep snare hit, low velocity (ex. Route 1)
10: High-pitch 256-byte sound (ex. Bicycle)
11: Soft snare hit, long; abrupt end
12: Soft snare hit, medium; abrupt end
13: Soft snare hit, short; abrupt end
Everything else above that is used either for Pokémon cries or attack sound effects.
In saying that, I'm highly doubtful that the GBA's GB Synth can replicate these specific noises with particular MIDI notes. They can be recreated, but (most likely) not played with pre-defined corresponding notes. Now comes the question of, "Where in the ROM can we find definitions for these particular sound outputs for noise instruments? And will they be in a form that can be converted painlessly enough to something that can be used in GBA games?"