AFAIK nnedi3_rpow2 doesn't support Stack16, but you can use
ConvertToStacked and ConvertFromStacked to convert between native 16-bit and Stack16
csp invalid? Make sure you have the latest version of DitherTools.
csp parameter is in the docs.
The but with AvisynthShader is in properly detecting 16-bit clips in the Upscale command.
Perhaps this will work, haven't tested
Code:
ConvertBits(16)
SuperRes(3, .43, 0, """nnedi3_rpow2(2, nns=4, cshift="Spline16Resize").ConvertToStacked()""", lsb_upscale=true)
ConvertBits(8)