I think this would be a good addition to the existing activation blocks. You generally must address them one at a time when building anyway, so I would suggest a configuration option ("R" or whatever button you have mapped to activate/use a block) for all existing activation blocks (walk, punch, and area), which brings up a menu/selection interactive screen for that activation block, under which you could activate the option to use in flight mode. This hotbar option would be off by default, so that you wouldn't have to sort through all your different activation blocks while setting up your flight-mode hotbar. This way, you could have a lot of different configuration options per block, and you wouldn't need to use up any new block IDs.
Personally, I think all blocks should have a configuration menu in build mode, which leads to the various options that we use the activation button for right now (normally mapped to "R"). (E.g. Use this block for weapon emission, Change AI to Turret mode, Turn off this light, etc.) Such a menu would give us more options and a lot more flexibility for future block changes.