> The simplest thing would be to use MIDI semantics but with float values.
> Thus key 60.5 would be 1/4 tone sharper than C.

Please don't.
For the most part, pitch included, MIDI doesn't have semantics.
It would be wrong, therefore, to equate "60" with "middle-C" or
a unit increase in keynumbers with one tempered half-step.

Also, as others have pointed out, there is no notion of duration 
in MIDI, hence no such pfield to set.

If you want a sequencer for csound, look at cm.  It doesn't care
about pfields, as long as you (the programmer) defines their
semantics.   I dont' see how a different approach is possible for