Will the Epic 645 hava a OLPF?

[Daniel Browning]

with 6micron photo sites and stops above f11 is the fuzziness due to diffraction enough?

For most people it would be enough, IMHO.

Is it possible to make a OLPF big enough for the 645...

Yes. It's only money. (Cost rises exponentially with area for the good crystal ones.)

At f/11 the diameter of Airy disk (the smallest point on which a beam of light can be focused) is 17 microns.
[...]
It's more than enough. The smallest detail the lens can focus at f/11 would be 3x3 pixels large.

I think MTF is the best way to measure the effect of diffraction. The higher the frequency, the lower the MTF (i.e., smaller pixels have lower MTF at Nyquist due to diffraction).

To avoid aliasing, an MTF of 0% would be the theoretical ideal, but in practical situations I don't think it's necessary to go that low. I think 10% MTF is sufficient to prevent aliasing. Using the formula for MTF of diffraction[*], I calculate that MTF drops to 10% for green light (550nm) at f/11 at 3.7 micron pixels. Using red light (750nm) I get 5 micron pixels. Both of these are slightly smaller than the 6 micron pixels in the EPICs.

Again, it's a question of what MTF you consider aliasing taken care of.
[*] 2/pi*(acos(f)-f.*sqrt(1-f.^2)) where f is 1/(w*f-number).