QTPlasmaC with six axis

If you pay attention to industrial beveling plasma machines, they all use a different torch holder with 4Th or 5Th axis, the point being that the torch can move but the nozzle stays at the same place during motion, well a bit under the nozzle at the cutting height.
That is done to avoid using 4 or 5 axis CAM software, as Rod mentioned, complicates things a lot and costs a fortune for software that usually does not work for what you need, or makes you jumps through hoops to get there.
Still, the question no one asked (or i missed it, glasses foggy) , why 6 axis?
Almost all are 4 axis, except Kjellberg Finsterwalde that have full 5 axis ones. Just do not ask for their price!!!
I have seen one with "nutating" head, the 45 degree spinning thingy, looked very interesting...

Found it

My vision was that 5 axis kinematics would not be not required and the bevel cutting would use the normal postprocessor with just the bevel angle set.

I think you'd be able to do this using the 'drag knife' feature (which I think Sheetcam offers) together with the 5axis XYZAC or XYZBC kinematic (TCP) in linuxcnc.

Not what is required here.

Well it may not work for your case, so apologies if I'm not being totally on topic but you have not really given much information about what you are actually trying to do other than '6axis complex pipe cutting' which could mean a lot of things.
@ Rodw
Since 'Drag knife' gcode orients the c axis (ie the torch in this case) to the tool path it may work for sheets and probably also for pipe. You could define the bevel angle using the A/B axis and then the C axis would be used to keep the torch oriented to the tool path. 

If you have cloned LinuxCNC then you should be able to checkout the branch with: