I finally managed to change GFX0 to IGPU in DSDT and actually see this in IOregExp. It turned out I wasn't using my edited DSDT after all, in fact I can't boot with it! I still don't know why this is, because it is patched by Clover itself. When I use an clean DSDT extracted with MaciASL however, the system can boot just fine. I guess Clover is somehow interfering here. Anyway, I now have an environment where I can try out DSDT edits.
Just for the record, I didn't use EZFlash to flash the modified BIOS, but FPT mentioned in the post.