Thanks for the advice. The problem is that I need more flexibility to load multiple files from custom folders, but I think I'll be able to do this using Node-WebKit's ReadFile().
The problem is that I really need to be able to load the graphics of Tilemaps through events, so I really miss "Load image from URL" for Tilemaps.
As an alternative, I experimented with generating custom tilemaps of small areas on a canvas using Javascript, and then load the resulting image on large Sprite objects. This would be no problem memory wise as all content is procedurally generated on the fly, but it somehow was much slower than using the built-in Tilemap object. This was partly because I needed to convert the canvas to a data URL before loading it into a Sprite.