If you have the joycon turned around so that it's still upright but the IR pointer is pointing towards the screen, then the only buttons clickable by parts of your hand other than your thumb would be:
* The R button or ZR trigger, but they're at the bottom of the joycon now -- you'd have to click them with the base of your hand, where it meets the wrist, or maybe your pinky.
* The mini-L and mini-R shoulder buttons, but they're on the RIGHT side of the joycon now, so you'd have to press them with the inner portion of one of your knuckles.
While there are plenty of buttons to emulate a Wii Remote, there just doesn't seem to be anything within reach of your fingers that can easily simulate the B trigger in a way where it can be pressed while your thumb is busy on the buttons normally required for the thumb (dpad, which the stick would emulate, and the face buttons). The only buttons available that you could map to the Wii Remote B button such that your thumbs wouldn't need to move would be the mini-L and mini-R on the left joycon, accessible to the minor fingers of your left hand. I think that would technically work, but it's so different from the Wii way that I don't know Nintendo would ever set things up like that.
Edit: If I was misreading your use of "upside down", and you meant that the analog was literally pointing at the floor... that would make it a really cool trigger, actually, but we'd definitely be at a loss for buttons.