Bladed spokes have been proven to help provide more lateral support to wheels however, 20 inch wheels are inherently stiff because of the short spoke lengths an heavy duty rims so the benefit would be negligible... Even the lightest bmx rim is more solid than many heavier 700c rims. More importantly while the Rays are very strong because they are entirely forged they lack the cross-sectional strength of a 2.0 or 2.0/1.8/2.0 Double Butted spoke, so they are more likely to fail because of contact with a ledge or someone elses pegs.
You may be able to find CX rays in sub 200mm lengths but you will more than likely have to either wait a few months to get them direct or hunt somewhere that has built some 20 inch handcycle wheels with them. I have only built 24 inch wheels with bladed spokes.
You can only cut bladed spokes where they are round so about -10mm can be taken off of them.
Rays will fit in a standard bmx hub as they are 2.2/2.3mm wide and hubs are drilled to 2.5+.
I would say that unless they are going on some race bike with carbon 20s its not worth the while...