Just thinking out loud, but you should be cognizant of this.
You noted that you are parsing and using DCGs which are great at parsing. However with executable code it can be context sensitive instead of context free and this can get you into a problem if you are not aware of it and know how to deal with it, especially with DCGs.
In other words if you do not have a formal grammar that describes the 6502 executable format that is context free and which is easily translated into DCGs, then might I suggest that you switch to reading the data as bytes, analyzing the bytes to figure out the displacement, size and types of the current instruction for use in accessing the offset and number of bytes of the next instruction, data, or meta data encoded in the executable file format. 