I highly doubt there is enough volume right now to make it worthwhile to do it by hand.
The one place where it could work is in the BitUSD/BitCNY market. Since fiatCNY is essentially pegged to fiatUSD, the ratio between BitUSD and BitCNY should change quite slowly over time, so manual market making in that market would be equivalent to placing a single bid and ask, +/- whatever spread you want to offer.