I'm working on a new version, hopefully a physical one
Since the rom is inmutable, you have to copy the map content into ram. The main problem is that it should fit (that is why I mentioned the map size limit)
edit: And thank you for playing the game :)
Thanks for the nice words Max.
The effect is a raster one. I am modifying the Y scroll each line using a table with the desired effect Y delta values.
More info about other effects here:
It is for MegaDrive, but some of them can be used in other tiled/sprites video game consoles.