I did that with my cabin staff that I actually met, but this was for the other staff that I didn't get to speak to.
Without sounding posh, I gave my Cruise Butler a copy of my book and asked him if he preferred fiat or Sats. He asked for Sats, but he is not allowed to carry his personal Smart phone in public areas onboard, so I wrote him a note on how to redeem via WoS, which he will do when he goes home on the 22nd November.
Also I tipped at the value of Sats at the begging of the cruise, not at the end, so they will receive more if they redeem them than they would had they received fiat.
Also, from an American tipping culture perspective, they already receive the tip part of their salary which is built into the fare we pay.
This is a tip above the default tip.