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.