Ok, apparently alRemapColor uses Luminance mode "Rec709" in Nuke. The actual formula is available in Nuke SDK:
r*0.2125 + g*0.7154 + b*0.0721
Arnold's saturation is what's called "Maximum" mode in Nuke, but Arnold has "Luminance' mode in aiColorToFloat - that does exactly the same as the Rec709 formula above. So, mixing it with original node gives us Nuke's Rec709 Saturation.
Thanks me 
