The potential difference between primary and secondary wiring of an isolating transformer
What voltage is a multimeter going to show when we put one probe into
  mains ground, and the other one at the output of the isolated
  (secondary) transformer wiring.


Theoretically 50% of your mains AC voltage, providing the multimeter has infinite input impedance and the secondary is disconnected from any load.

This value also assumes that there is no grounded shield between windings and that the net electric field is evenly distributed in one layer of windings.

All manner of other subtle issues can raise or lower this figure because it is based on inter-winding capacitance. For instance, because it is likely that the primary and secondary are wound using multiple layers, the main electric field imposing on the secondary will tend to be more towards ground potential or more towards live potential. This is dependent completely on which of the mains wires connects to the layer closest to the secondary. Swapping live and neutral on the primary could easily give different results.

Should this measured voltage be a worry? No because it is sourced through the inter-winding leakage capacitance and this will be in the order of 100 pF to 1 nF (hand waving alert!).