Depending on what you want to make the Miasma yield, it might be possible to cheat it in some ways - you an use the LUA to cycle through all applicable cities (based on which ones can get yield from Miasma), then for each city, cycle through all tiles it owns to keep a running count of the number with FEATURE_MIASMA on it. At the end, Grant yield appropriate to the number of Miasma tiles. It won't show up on any of the tooltips, sadly, and unless it's food it will be granted to the player total instead of stopping at the city first (so no multipliers, no using for culture expand), but still potentially useful.
