Nice questions.
-no chip in the slot?
It boots up the Super System Card 3.0 screen.
-a TG-CD System Card 2.0?
I'm pretty sure it boots up the System Card 2.0 screen. I haven't tried it myself, but I'm fairly positive on that because I remember people who do this so they can check out the cool wrong system card screen for games.
-a Super System Card 3.0?
Again, I think it boots up the card.
-an Arcade Card Pro (using a converter or region-modded system)?
I know a few people have down this, nodtveidt in particular, and I think it works overall. On the other hand, it seems like I recall there being problems when booting it up with a converter.
And also, has anyone actually tried to use the Arcade Card Duo with the TG-CD? I'm assuming you get the same "not enough memory" message you'd get with versions 2 or 3, but I'd just like to know if anyone's tried.
I think this also has been discussed here before. I wasn't aware there was an actual message that says "not enough memory"; that's pretty cool!