Speaking for bitcoin-central.net
Q1: Do you offer Market Maker accounts at the exchange?
Everything is negotiable.
Q2: Do you offer the ability to keep multiple fiat currencies in one account? (USD, EUR, GBP, JPY, CHF, AUD, NZD, BRL)
Q3: Do you pay interest on fiat cash balances?
Q4: I would like to do the fiat crosses using my current broker(s), this would mean that there might be multiple deposit/withdrawals per day. Would you allow this, and if so, what would be the holding period before the deposit is available for trade?
Usually, wires are credited the day after they're sent
Q5: Are sub-accounts allowed?
They're not implemented.
Q6: What extra benefits/rewards would you provide Market Makers for providing liquidity?
Free internets and karma points.