This pretty much sums it up as there seems to be no other way than to connect output ground to input gournd (i.e. Protective Earth).
at the scales involved the 125 KHz signale are basically magnetic fields, so to disrupt a card from 1m away you need to make a stronger 125KHz magnetic field in the proximity of the card than the card is making (or the same sort of signal level at-least), that's probably going to require large coils and strong currents.
UL - is only concerned if you sell the product. If you design it, build it, and it hurts you then everything is your own fault. If you sell it to a consumer then the UL gets involved to keep the consumer from getting hurt. Note that I said consumer. A consumer is an average person with no technical knowledge what so ever. In an industrial setting, there are ...
It is also possible that you change between different antennas using a reed relais, I'm not sure if that's what you do with your multiplexing. At least some RFID readers are fine with disonnecting the antenna mid-run which would save you the time of power-up/power down.
In that case, the reader should return a new tag each time the antenna (& tag) ...
It always helps to find the datasheet of the parts you use:
That's from the datasheet. It says there's two types of reader:
Read once, must move card out of range to read again.
Read continuously - card data will be returned as long as it can be read.
You seem to have the first type, which is the standard type.
If you want to read continuosly, you'll ...
The following is worth a try and won't cost much.
Grab a length of cheap speaker wire - the "zip" kind of wire that consists of two parallel conductors that can be easily split into individual wires. Make it as long as the bench.
Solder a 50 Ohm surface-mount resistor at one end of the wire across the two conductors. You can use a through-hole resistor ...