I connected a 12VDC transformer + to the wire going to the garage door opener.
You can have a 12VDC power supply.
You can have a 12VAC transformer.
But a 12VDC transformer, no.
Where did the 12VDC come from? I thought the panel only had 24VDC?
I'm going to assume you have a separate 12VDC supply in the garage door opener.
The return wire from the opener, I have going to the positive side of a 12-24VDC blue led which I believe implements a miniature step down transformer.
I highly doubt there is a tiny transformer of any type inside that blue LED. More than likely it simply pulls twice as much current at 24VDC as it does at 12VDC, but both are within its specs.
In parallel, I have jumpered a wire to the twido input card.
The negative side of the 12VDC transformer I connected to ground and the negative side of the led.
So basically, you put a light across the input, no big deal there. Has nothing to do with matching 12VDC signal to 24VDC input, but it doesn't hurt anything either.
If your 24VDC input will fire with only 12 volts, hooray! It's fine. It would be nice to know how many volts it takes to turn on the input so you could be sure it would be reliable. (i.e. if it turns on at 10 volts, you're good)
There is obviously some minute amount of current going from the 24VDC transformer to ground.
I am wondering if I should ground the 24VDC - terminal.
How bad is this?
The 12VDC and the 24VDC common should be connected together. If they are both grounded at the same point, they are connected. If they are grounded in two separate locations, see if you can ground them together instead.
Jeopardizing the UL label? I don't think you have a UL label anymore on the garage door opener or the PLC panel.
If I had to so something like this, I think I would approach it like this. Try to minimize modifications to both systems. Don't directly connect them!
Wire a small relay coil to the garage door opener, have the coil pull in with the opener on its power (12VDC).
Wire an isolated contact from the relay to an input on the Twido using its 24VDC power.
Use a UL listed relay.
You may still not be 100% kosher with adding the relay to the garage door opener (its UL label), but you should be fine on the PLC panel.
You can probably even mount the relay in the PLC panel without causing trouble.
Now you don't have to tie the 12VDC and the 24VDC together at all. And no wires directly connect the garage door opener circuits to your PLC.