I've only Hanabbed it up a handful of times (got 25 w strict play, no mistakes w my brother over Christmas!), but I've been of the conviction that there is too much emphasis on these "conventions," which either require a pregame explanation and agreement, or more often in-game argument (which is against the rules). I prefer an emphasis on card holding that constantly displays your knowledge of your own hand. To me this isn't against the communication rule, because the cards have to be on display. They can be held sideways, higher or lower, upside down (The cards are orientable on both faces, people don't notice this!), I hold them between certain fingers to remember number values. While that's just another convention, one can observe it develop from the beginning game state, and every time a player receives a clue, how they redo their hand. In summary, card holding is very effective/information rich and can be deduced without words, and seems to me perfectly natural to the game. This also relieves a lot of the cognition devoted to remembering what other people know about their own hands. When discussing a UI for a digital version of Hanabi, my brother and I discussed mostly how customizable the card holding would be, which would be exactly reflected to the other players.