Here's a thought:
Base a PC's PP (that sounds vaguely...illegal) entirely upon Con for all manifesting psionic classes (risky, I know, letting such a high-value stat double as a casting stat), but have all Manifesters have to deal with a fatigue mechanic, not with an ASF mechanic. Concentration may or may not apply, depending on the particular power.
After a certain % of a Manifester's PP have been expended, he has to make a Fort save each time he manifests a new power (DC based on the power's level) or become Fatigued. After the next % threshold, he becomes Exhausted on a failed Fort save. The third failed Fort save results in Unconscousness and the failure of all powers that require concentration or direction.
The Soulknife (and other non-Manifesting psionic classes) would still need PP for their abilities to be active, but wouldn't be subject to fatigue.
What this does:
1) It closely mirrors what happens to psions of the fiction that inspired their inclusion into RPGs. Most of them (that I've read about or seen in TV or film) become progressively more exhausted the more power they exert.
2) It explains why Manifesters aren't affected by armor.
3) It imposes a direct and immediate penalty on Manifesters who go Nova, but still allows them the option of doing so if they really, really feel it is necessary.
4) Because of this fatigue mechanic, it might make sense for all psi classes to have a few more skill points. By and large, they're going to want to be more resourceful than other supernaturalists, if for no other reason than they're not going to want to pass out on the floor in front of a bunch of Worgs.
5) The distinction between manifesting and non-manifesting psionic classes becomes more meaningful, and would even more closely resemble the distinction between traditional arcanists (the Wizard, the Sorc, etc.) and the Warlock.
6) It would also make meaningful the distinction between the non-Manifesting classes and Manifesting classes that are somehow able to mimic the abilities of the non-Manifesters.
IOW, if you did have a base class Soulknife, but also a mindblade power accessible by other classes, the Soulknife could keep his active all day long...but the PsyWar with a mindblade would, over time, feel his consciousness fade and fade and fade...
7) A Manifester's overall health would directly impact his manifesting ability- again, consistent with the underlying fiction. A spell or disease that affects Con, starvation, dehydration or any form of exposure- all of these things would make it more likely that a Manifester would fail one of those Fort saves. And if the other condition imposes a condition change in and of itself, the Manifester is likewise hampered.