random mask reversible after homomorphic encryption I would like to know if this process is feasible under homomorphic encryption, ideally under paillier or any other additive scheme 


Apply a mask X to obfuscate a message A ie. Am = A (op) X where (op) can be +, x, ...
Homomorphically encrypt Am => E(Am)
Remove X from E(Am) in order to obtain E(A) without any decryption (without knowing the private key)


A can be a matrix, vector, ...etc.
It sounds like you're looking for a way to do blinded encryption (that is, encryption in such a way that the encryptor does not know what he is encrypting).
It is certainly possible with Paillier.  Here is one way:

The (op) will be multiplication modulo $n$ the Paillier modulus (which is in the public key).

To mask, we pick a random $r$ relatively prime to $n$, and compute $Am = A \cdot r \bmod n$.  We can see that, if we selected $r$ uniformly, then $Am$ is distributed independently of $A$ (except for values not relatively prime to $n$)

We ask the encryptor to take $Am$ to compute $E(Am)$

To unmask, we compute $rinv = r^{-1} \bmod n$, and then homomorphically compute $E(rinv \cdot Am)$; this Paillier, this is done by computing $E(Am)^{rinv} \bmod n = E(A)$.


One drawback to this is that it doesn't actually mask the value $A=0$; is that a concern?
If it is, you can still do it by asking the encryptor to perform two masked encrypted operations.
First, you generate two random values $r, s$, making sure that neither is $0$
Then, you have the encryptor encrypt the values $A-s$ and $r\cdot s$, producing $E(A-s)$ and $E(r \cdot s)$.  Then, you use the above procedure to recover $E(s)$ from $E(r \cdot s)$.  Then, you homomorphically add $E(s)$ to $E(A-s)$, producing $E(A)$
You could just generate $E(s)$ yourself with $s$ and the public key.  However, if you could perform encryption yourself, you could just generate $E(A)$ from $A$, and it would appear you wanted to avoid that...