I'd like to change the MTU per host as I have a mixed infrastructure where not all devices support the same MTU. I'm on OS X 10.7 and like to set some MTU per destination host and a default one.
As the others pointed out in the comments, MTU is set per-interface, and the only "per-host" capability available is Path MTU Discovery (PMTU).
OS X has the sysctl knob for Path MTU Discovery -- It appears to be on by default in Lion:
so things should be pretty much self-tuning.