I've joked about this before but was half serious. 
Like you, I'm not holding my breath, but it seems a lot of new scene switches/dimmers/remotes support up to 5 taps, and this would be a clean-ish way to accommodate them without needing to do button-number arithmetic (my preferred workaround given the circumstances), awkwardly usurp button attributes for odd purposes (Inovelli, I love you, but...), use custom attributes/events (not really usable outside RM or a custom app), or forget about them entirely.
And I understand where they're coming from, but I am indeed capable of remembering 5 taps if I use sufficient mnemonic devices (taps up are vaguely "on" for me and correspond to either that or cooler temperatures; down taps remind me of "off" and are for that or warm temperatures, and the fifth tap that is maybe otherwise hard to remember is often a mode change--down for night, for example, again reminiscent of "off "), which also makes it easier to remember. I'm totally fine writing custom drivers for what I need, but this is a question I've seen from new users, too.
But perhaps a more generic approach would be better: "scene number" and "key attribute" might be a bit too Z-Wave-centric for a platform event, but "quintuple tappable butron" also means lots of capabilities just for this...all probably part of why they've never done this. But I can dream.
