[conclusions from my own research]
I opened the upper shifter cover to study the release mechanism. It looks like it's fundamentally different than in Shimano shifters, and implementing a multi-release action may be impossible.
In Shimano the release lever is pushed forward (rotated around the shifter center), and then it releases one click on the shifter index. When pushed further it just releases another click. So "tuning" 9spd XT shifters to have this just required removing a small piece that was blocking the lever after first click.
In SRAM shifters, however, the release lever doesn't move forward, but turns upwards, using this movement to slide a metal plate inside to the left. This probably releases one tooth on the index cog below the cable spool, but then blocks the cog from turning further on the right side (as the metal plate is still pushed to the left). When you release the lever, the plate slides back right, the central cog rotates a millimeter more, and blocks the cog again on the left side, as before the shift.
I'll add more if I find out anything, cause I don't want to experiment on my only shifter :) To conclude, it's probably impossible:
- the metal plate blocking the index cog has to move left and back right for a complete relase cycle
- there is probably no room for the lever to turn upwards further than it does now