Greetings,
I think Solarus should have tabs for tilesets. This is one way to get around the GPL and creative commons mixture issue. GPL and CC can be used in the same project due to art/audio being nonfunctional data based on beauty (All the programming code becomes GPL, not art.), but the artwork being on the same png might attract the extreme license people. I doubt most of the creators will even care as long as the CC and the GPL art are not being added on to each other directly. The way to get around this is by having the art on different layers.
For example, let us go with a gpl trunk and a CC-BY-SA leafy top. In Solarus the art can be on a GPL sheet and the other on a CC-BY-SA sheet. That way it avoids license issues with a single PNG. Solarus can overlay them to make a single tree.
Visual example: (Ace has tabs for tileset sections (ABCD), but it is not exactly designed for avoiding license issues.)
(http://www.johnwizard.com/img/articles/vx-ace/image_tileset.jpg)
~Zefk