Ruths suggestion of a tray is good idea, i would leave the other trays(s) as normal and put an extra one there.
If shes only doing it on the door mat, to me says shes worried about intruders comming in. Is there a cat flap there?
Have you got feliway? A plug in and then spray the area would help.
Maybe shutting her in one room and starting again with intros of the rest of the house, if your room lay out is suitable.
I use simple solution or a bio powder followed by a spirit.The bio powder would please your OH.
If she doesnt stop soon i would consult a behaviourist before it becomes a permanent habit.