Ok, so basically use the diagnostic card as though it was a normal id card, and then place the xFD over the LF coil of it (I believe its the circular one in the centre) - basically we’re trying to recreate
but LF. Honestly, I have no idea if it will work the same with LF / x series antennas, but it can’t hurt to try as long as you do it stealthily.
One reason I’m thinking it could work, is that the datasheet suggests 60ma standby and 155ma max current - maybe it increases power when it detects a card.
Not sure if you tried it yet, but this suggestion from pilgrimsmaster is similar, and I’d try that too: