for those interested, Hans has implemented in latest beta from 2015.12.22 10:50 values for duplex printing in the option key from \setupinteractionscreen.
With this commit, it is possible to disable duplex printing in printers which have it set by default, or enable portrait or landscape duplex printing (no matter which the defaults are).
More information at http://wiki.contextgarden.net/PDF_Print_Options. I must admit it is formatted like crap. But I had to write it in Markdown and convert it to mediawiki using pandoc.
I hope you enjoy this feature,