Option One: Multiclass Rogue
This one initially looks pretty straightforward. Build your psion the way you want, take the Sneak of Shadows multiclass feat, and use Sneak Attack once per encounter.
The difficulty is that Sneak Attack can only be used when you attack with a light blade, crossbow, or sling. I'm not aware of any way to get slings as implements. You can get light blades as implements by being a hybrid psion|swordmage (they're both int-based and swordmages have some stealth options, so a build like that might be at least somewhat viable). Alternatively you could get crossbows as implements by going hybrid psion|artificer and taking the Crossbow Caster feat, but hybrid swordmage is likely to be better both from an optimization standpoint and in terms of how well it fits your desired playstyle/concept. Either way it's a lot of hassle to go through just to be able to Sneak Attack once per encounter.
Option Two: Hybrid Rogue
This is... not a good plan. You give up several power points to get some rogue powers that won't be useful unless you've built to keep your dexterity high enough to attack with competently, and dex/int is not one of the better ability score pairs to boost. You get Sneak Attack once per turn just like a real rogue, but you can only use it on those rogue powers you took, not your psion powers.
Rogue and psion just don't have enough synergy to make this an attractive choice; hybrids end up being worse than single-class characters 95% of the time, and this build doesn't look an exception to that rule.
If you do go this route, Sneaky Staff is a strong feat choice; it allows you to use staves for Sneak Attack and rogue melee powers, which solves the problem of having to get enchants for a weapon and an implement separately. Its benefit only applies to weapon attacks, however, so it's of no use if you merely multiclass into rogue.
Option Three: Radiant Mafia
Now, you may be thinking, "Radiant Mafia is a divine-focused party optimization, what does that have to do with being a sneaky rogue?" The answer is, not much. Or at least, not much to do with being a rogue; the sneaky part actually has some synergy with some things that are popular with Radiant Mafia types.
Now the caveat here is that these synergies aren't available until epic level. The linchpin of the combo is the Radiant One epic destiny, the level 21 feature of which includes (but is not limited to) dealing extra fire & radiant damage equal to your int modifier (psions' main ability) whenever you deal damage to a target you have combat advantage against. What's an ideal way to get combat advantage at range, especially against multiple foes? Stealth. Who has lots of AoE powers to use against multiple foes they have combat advantage against thanks to said stealth? Controllers, such as psions.
If you're willing to tweak your character concept to focus more on sneaking around so you can jump out from behind bushes shouting, "Behold!" and then dazzle foes with your awesomeness, this build can work pretty well.
I won't go into optimizing such a build here, but since you'll ultimately have a reliable source of radiant damage, you can benefit from building around a lot of the standard radiant damage combos. Getting radiant damage before epic to take advantage of them can be trickier, but psions get staves as implements and I think the Radiant Weapon enchantment can be applied to quarterstaves, so there you go.
TLDR: The primary benefit of stealth is combat advantage (and surprise rounds for alpha striking, I suppose). Rogue's Sneak Attack is the most obvious way to get an extra boost from combat advantage, but it's not the only way.
Option Four: Refluffing
Have you considered wizard instead of psion? Wizards have a number of mind-affecting powers, have summons you can use as decoys, and are also intelligence-based. They also have a lot more support published, and are a lot easier to optimize. Getting light blades as implements is as easy as taking Arcane Implement Proficiency. I'm pretty sure they have a PP focused on invisibility as well.