I mean one "pro" mixer rather than several mixers. I mean, sure, if there are several mixing services around why not join them together, but I doubt that'll happen for a while.

Running a mixer is risky because various government regulations in US/EU require you to know who your customer is once you're doing transactions of above a certain level, I think. See here:

http://en.wikipedia.org/wiki/Anti-money_laundering

In other parts of the world it's less likely to be an issue, but given that AML is quite tied up with the "war on terror" it's worth pondering the risks before setting one up.

For the database it's possible to obfuscate such that it cannot be read but as the entries should be transitory that won't provide too much value. Consider this scheme

Columns: KEY, VALUE, A, B

A and B are large random numbers. You want to insert (foo, bar).

KEY = hash(concat(a, foo))
VALUE = hash(concat(b, foo)) ^ bar

Now key and value are both obfuscated. You can read the value only if you present the key. If the keys and values are BitCoin keys they will be difficult to brute force.

That said I'm not sure that pattern can easily be applied in this case.