Is there a way to 'paint' or 'fill' tiles in manyland?
Yes, you can quick-fill areas in most locations, and at most ranks. To do so, find an empty space and and place tiles in the following order:
1 2
4 3
or
2 1
3 4
That is, only the 4 edges, in clockwise or counter-clockwise order starting at the top. This will work with many block types (like solid, back, liquid). A video of this in action can be seen here.