1920 rowhouse project - rewiring
Put a junction box in a location the wiring can reach.  Make a splice inside the junction box to get to where you need to go.  Splices are no big deal if done right - really.   

This location must be accessible without dismantling any part of the house.  So it can't be behind trim or drywall.  But putting it inside a closet with floating shelves is fine.   If all the person has to do is empty out a closet and lift out a shelf.  

The junction box will have a blank cover, unless it's in the bathroom. 

Or, if it really makes you bat crazy to have a blank cover, then fine.  Use a deep box at least 20 cubic inches.  Make the bathroom splice and push it into the back of the box.  Then bring over another circuit and attach a receptacle to that circuit. That's allowed as long as the box has enough cubic inches to handle all the wires.  Don't mix the neutrals.