Citi, UBS and Deutsche are names that are bandied about, though these are unconfirmed.
The reserves in question amount to somewhere in the region of $1bn (25% of deposits) and bankers in San Salvador say that under the laws that paved the way for dollarisation, the banks should be allowed to manage the reserves themselves. A central bank without a currency is, of course, a rather forlorn institution and there is a sneaking suspicion that keeping hold of the reserves is a way of staying in business.