Is it possible to build a plugin for loading image data (for the tileset) from a non-standard compressed format?
If so, is there any documentation for doing this? Or perhaps some code floating around that does something along this line?
Image format plugins are outside the realm of Tiled, but it is possible to add a Qt plugin that will allow Tiled to read your images. For this you need to implement QImageIOPlugin. Maybe this Qt Quarterly document could also help, but be warned that it will be out of date in some areas since it was written for Qt 4. See the plugins howto for more up-to-date information about writing Qt plugins.