I can't quite get the wandering trader to work, so I stopped on a zombie, and it... seems to work. Not perfectly, but well enough. I place redstone torches beneath the door to open it when a zombie gets near.
Now I only have one problem. If it opens the door, it's possible to quickly close it, and it will stay closed (since the zombie didn't move away and reset torches underneath)
I think I'll try making a timer to replace torches, but it will make a lot of noise, so... I really want to find some prettier solution to this