Can natural weapons be concealed with a disguise self spell?
Yes, because from Disguise Self,
You make yourself—including clothing, armor, weapons, and equipment—look different.
Would it be allowed to make sneak attacks with its face-tentacles in this instance, because people have no way to see it coming?
Probably not if you're already in combat.
Disguise itself makes no mention of sneak attack. If you compare the tactic with the skill Sleight of Hand, (since the weapons are hidden), it reads,
Drawing a hidden weapon is a standard action and doesn’t provoke an attack of opportunity.
What it doesn't say outright is what the benefit of drawing a hidden weapon is. The Complete Scoundrel added a skill trick, Hidden Blade that makes it a move action and grants sneak attack. This is pretty strong evidence that the original usage does not grant sneak attack, but that's probably up to the DM.
The Rules Compendium also adds "Dagger Surprise" which works similarly to the Hidden Blade skill trick, but both have a dependency on Quickdraw.
A DM may allow it if instead, considered it more like feinting in combat from the Bluff skill,
You can also use Bluff to mislead an opponent in melee combat (so that it can’t dodge your next attack effectively). To feint, make a Bluff check opposed by your target’s Sense Motive check, but in this case, the target may add its base attack bonus to the roll along with any other applicable modifiers.
If your Bluff check result exceeds this special Sense Motive check result, your target is denied its Dexterity bonus to AC (if any) for the next melee attack you make against it. This attack must be made on or before your next turn.
As it's not too big a stretch from disguised weapons to misleading an opponent, but again, up to the DM.
However, if you are not in combat, Yes
KRyan points out, if the disguise is used outside of combat, and the disguised character successfully pretends to be someone else, known to the defender as friendly or at least non-hostile, that would conceivably afford a surprise round, which would cause the defender to be surprised, and therefore flat-footed. If the disguised attacker has a higher initiative, that's flat-footed both for a standard attack during the surprise round and a full round action in the first round.
As HeyICanCHan points out, it does require a broader view of "aware combatant" than what is outlined in surprise. It requires making the leap that fooled would equate to unaware, which is reasonable, but not strictly mentioned in the rules.