Hey Adrian, that's a great initiative, thank you for that. I was trying to implement it but I didn't have enough knowledge to pull it off :)
I think your extension still needs to generate the tile map array in the header file, right? I found this implementation that does that and I was trying to adapt it, but it seems that a loop through every single pixel of the image is needed, to get its colour and convert it to a byte to be used in the function set_bkg_tiles, for instance. Would you happen to know how to do that?
I get the impression that I misunderstood what you actually needed. The extension I have developed does a similar function to GameBoy Map Builder and not to GameBoy Tile Designer.
That is, my extension should generate the tile map file, but not the tile graphics data.
In my workflow what I do is to design with an image editing program (for example, Aseprite) a tile tileset with size 8x8 pixels (the one accepted by GameBoy) and then export it to a png file.
That png file can be used to generate a tileset from Tiled and at the same time it can be imported into a tool like gbtdg to get the graphics tile data in C.
From there, you can from Tiled generate the map and export it to GBDK C format with the extension.
I hope this has cleared up any doubts about its use :)