A little extra weight...

[Magicman]

I would never do it.  Any upward pressure on the up/down chain (caused by downward pressure being exerted by the head) will cause the teeth on this bottom bracket to bend upward and fail.


 
Notice the new bracket.

[4x4American]

;D

[Dave Shepard]

I still contend that using just the weight of the head is ok. Down pressure with the up/down motor would do damage as Magicman points out. Don't forget that the two plane clamp can also be used to pull down on a board or cant? ;)

[Magicman]

I still contend that using just the weight of the head is ok

I agree, just be sure not to pull down and add more than the actual weight.  Of course unless you have extended your power strip, (or something similar) you can not use this option anyway.


 
A large C clamp will also work quite nicely to snug a bowed cant down to the sawmill bed for clamping.