Are you allowed to park just before a pedestrian crossing if the street is one-way?

No, it is always prohibited to stop or park within ten metres of a pedestrian crossing. It does not matter if the street is one-way or two-way.

The reason for this is that a stationary vehicle just before a pedestrian crossing would obscure the view of other drivers approaching the crossing and reduce the time they have to detect someone who has or is about to enter the crossing.