[The Bald Man]

The mana-based invisibility spell works on the mind.  I envision it as a "ignore me, I'm not here" (like obfuscate for you VtM players). 
Wouldn't that work against astral observers?  If they are ignoring "you" they are likewise don't see the aura of personal effects.  Start extending spells beyond your aura and you have a different situation. 

[UnLimiTeD]

A solid argument, however, invisibility works on the visual sense only; It won't stop someone from running into you, or hearing you.
As Astral sight is an entirely different sense, it will obviously not be affected.
Let alone the fact that an invisibility spell floating around would be quite suspicious.

[Novocrane]

Though mana-based illusions can be created on the astral plane, their magical auras give them away as illusions to anyone who makes a successful Assensing Test—illusions can’t fool Assensing and cannot be used to disguise or create auras.

[Jack_Spade]

For completeness sake it should be mentioned that there is a specific spell that can create illusions that fool astral sight:
False Impression/Manascape from Shadow Spells p.20

It's implied that the spell also hides it's own aura (otherwise it would be pretty useless)