The keystone to making the system seem familiar (for BX players who need to quickly "get" how a psionic system can work easily with their system) was the idea of incorporating a mental class (MC), and rolling “to hit” with psionic attacks. This would be for psionic-vs-psionic combat only, though; "to hit" rolls are not needed vs. non-psionics, but non-psionics do get a saving throw based on their INT and the range of the attack (this is actually something that comes from OD&D). As for MC, I see it being based on the defender's intelligence and their level as a psioncist.
There would be a separate attack table for psionic attacks based on attacker's psionic level and defender's MC (and it would look just like a standard attack table). Also, combinations of attack and defense modes would just affect the defender's MC while in that mode (e.g., if an attacker were to use an Ego Whip attack vs. a Mind Blank defense, the defender would suffer a +5 MC penalty against that attack).
In this model, any monster with psionic powers would have to be given a psionic level specifically for psionic attacks/defense (in addition to its normal HD/AC, which would still be used for determining physical attacks). This BX system would still need some sort of "psionic energy points" in addition to level determination. I'm probably going to take a cue here from the 2e psionicist, with each level giving the psionicist about 10 additional energy points (give or take a few based on WIS). Using this standard, a monster like the 1e shedu with a psionic ability of 70-100 would attack defend on mental attacks as a 7th-10th level psionicist (which seems about right for a 9+9 HD monster). The su-monster, on the other hand, while only having 5+5 HD, would attack as a 12th level psionicist (since it has a psionic ability of 120).
Ultimately, my goal is to be able to quickly convert 1e psionic information (particularly for monsters) into a simple to use BX system (that could be handled in less than a dozen or so pages).