Originally posted by sad_punk ... The image is still viewable in the viewfinder even when the lens is not cocked ...
That should not be the case. If you haven't cocked the lens shutter and you're using the lens in leaf shutter mode (that is you've selected a shutter speed on the lens barrel) you should have a blocked / black viewfinder until you cock the lens shutter.
It is not completely clear to my whether or not the Sigma flash fires. Your claim that is works in focal plane mode gives me the impression that it does not fire in leaf shutter mode.
Have a look at the section 'Using in Leaf Shutter Mode' of the LS lens manual. Using the lens on an original Pentax 645 I can vouch that the procedure how to use the lens is exactly as described. I also used this lens on a P645N and I am not sure if the camera switches to 1/8s shutter automatically. Have to check that.
I would recommend to check out the manual and make sure you'll notice the view finder blockage when you've tripped the leaf shutter. If you do not see the blockage, your lens in malfunctioning hence no flash output.
On a side note. You should be able to test drive this setup without film. Unlike the 645 the 645N fires when no film is loaded.
Good luck!
PENTAX SMC 645 LS 75MM F/2.8 OPERATING MANUAL Pdf Download.