3
A work-around would be to:
Create a user with the custom email address
Create a group
Add the user to the group
Set this group to be recipient of the form submissal
But I assume you have already considered that and found it to be too inconvenient - so if Formulaire does it, I would try it out. As far as I know, it is built on Liaise, so it should not lack Liaise's features.