Ian,
I can't help with the titration question - but we routinely use several
antibodies 1:5 -1:20 (sometimes even neat).  To save antibody, we use 10 -
20 ul of antibody and then cover with a soft plastic coverslip (cut up
squares of parafilm works fine).  I usually run a sample test with and
without coverslip to make sure staining is equivalent - so far I haven't
found an antibody for which I can't use this technique.  I borrowed this
technique from our In-Situ work which routinely uses a coverslip during
hybridization.  Hope this helps someone, if not you!
Chris Rogahn
crogahn@prius.jnj.com