I did in a few instances, and for reasons. I believe I did this at programs that said +1 was OK. I think some of them indicated that they would not be able to cover food/drinks for the plus 1.
I might have asked if it was OK for my SO to attend and pay for themselves, and I'm sure the way I worded it was compelling enough.
Something along the lines that my long distance partner was going to be in the area at the same time job interviewing and house hunting, because we were that committed to settling down in that area for both our careers, and that he would appreciate a chance to meet my potential future colleagues. That I also understood if they thought it best I come alone.
A lot of programs will also allow the residents to bring someone as well. It really depends on the details and vibe you get from how each program does their dinner. Some just sort of have a casual drop in happy hour night at a bar where anyone can come, vs others that have some fancy RSVP thing with dinner choices made ahead of time. That can be a heads up if you think you can ask without coming off wrong.
I remember working on this little explanation more to make it sound prettier and I didn't really try to do this except where it applied, as mentioned. I got mostly positive responses to this, IIRC. IIRC one place took a pass on it, but didn't seem offended I asked given why I asked.
