I have been thinking about how to extend the Z axis to be able to work on thicker stock.
And I came up with this idea.
By mounting the waste board on the bottom of the rail instead of on top you increase the max work thickness by 20mm (the thickness of the rail)
Though this does not increase the possible carving area on the thicker stock.
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If you replace to bottom rail with thicker extruded rail (20mm x80mm) you can increase this by 80mm, allowing you carve on very thick stock! (6X’s, logs, etc.)
(Possibly using the 20mm rails from the kit with some L brackets to cross brace on the normally open sides and help keep things stiff and square.)
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In fact if you got a second waste board you could leave one on the bottom for when you need to do thick items and have a removable one on the top, in the stock position, for everyday work.
I think it would be necessary to always leave a waste board on the bottom, as opposed to swapping a single board between both positions. Otherwise you would have to re-square up your x-carve every time. 
I also think the lowered board approach would be useful if you wanted to use a rotary axis. As only the top half of the work piece needs to be in the cutting range.
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But none of this increases the actual carve area. Adding a longer bit doesn’t help. It just shifts your carve area lower, reducing your maximum stock thickness.
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But if you extended the length of the Z rail and ACME screw so that the spindle motor can retract higher, allowing the longer bit to be lifted clear, it should work.
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I imagine if your carving geometry did not require the full depth of cut, just the full range, you could use a normal bit in a drill bit extender to give you the necessary length.
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Would this work?
Does anyone know if this approach was tried on the Shapeoko?