Hi there, sorry for the late response. I've been researching how to do C64 SEQ file export (and maybe import as well) and was having trouble finding info about the file format. I'm making some progress though. It will be available as an import "plugin" like the DOS ANSI file export etc. I'll either post the plugin file here or make it the main part of the 0.9.11 update. No ETA.
SEQ files themselves is just simply SEQuential data, nothing special there. For the C/G formatting, I don't think there should be anything special. It should just be each character to represent a C64 PETSCII character (space, cursor key, graphics, etc). Since you already have the PETSCII character set, the only thing I would figure is to see if those characters actually map up with the character codes.
Beyond that, I can't imagine much more difficulty. Just would need to make sure when you export that it actually does blank spaces. Since Playscii is a static image, you might need to recognize cursor keys for importing. However, exporting this shouldn't be needed. This should only be needed for the animated (movie) C/G SEQ movie files.
If it will help, I can see about tossing you a few files from my BBS collection your way to inspect. I can include both static and animated.
I can't seem to find a way to post files here (suspect you can't), so here is a link to download a small collection.
Inside, you will find a few C/G pic files (that on a C64 or in DirMaster, are correctly viewable). In addition, I made BMP images of each, so you can see what they should look like when viewed properly. I also included one C/G animated movie, just in case you find that interesting or useful (prob not as much for Playscii, but whatever).
Hope this helps!