We've found that the gay areas of the cities are best for meeting singles for threesomes. They arnt just gay people who go there, quite a lot of straight and bi people go as well. My husband is totally straight we just find it easy to meet single men for this sort of thing from going to these sort of bars and clubs if swinger clubs arnt your thing.
From my experience with your idea its very hit or miss. A lot of men in some way or another simply wont do it either because they believe its wrong or when they come to put the condom on they will go soft then ask if the husband can leave the room so he can perform. You might want to try just letting her be alone with him, she tell him she is cheating on you because it is surprising how many men get turned on from fucking a woman who is spoken for and cheating behind her partners back, when really she could just be doing it because he allows her to.
With some men ive told them im in an open relationship/marriage, they have always been cool with that.
Its after we get talking i see what his level of open/dirty mindedness he has, before i tell him my husband likes to watch or participate then i check his response then i might ask him if its something he is interested in doing.