I understand why Mark would prefer not to use Mentalism spells for psionics in Dark Sun, but personally, I would, because it is much easier to do than to invent a new system for it.
I would also say that using the Mentalism realm for Dark Sun psionics is even easier in the new version of Rolemaster coming out soon, RMU. This is because RMU essentially makes all spells of the caster's level or lower into Instantaneous spells (no prep necessary). You also don't need to make gestures or sounds to cast Mentalism spells. So RMU-style Mentalism seems even closer to Dark Sun psionics because of this (even if they are not exactly the same): they can happen more or less instantly, rather than needing a lengthy period of spell preparation.