I used to work in casting. We would take both big bundled submissions from agencies, and single submissions, get rid of all the envelopes, and go through all the headshots. (Thus losing track of whom came from where, we would refer to the resume info later instead.) A headshot from an agency we liked had an unspoken priority over a random submission.
So my question now is: Would it benefit an unrepresented actor to use an agency logo & address (obviously not one of the "big" ones) on their mailer? And then just have their non-agency contact info on their resume?
Would it be detected?
Would it cause offense if it were?
Thanks for any response!

I do not think so. I think if you do something dishonest it will come back to haunt you.

I'm trying really hard but I just can't imagine a worse idea. What will the casting person think when they open the envelope, expecting to find an extensive agency submission and instead, all they find is one picture from one actor???

This idea has bad news written all over it.

I have heard of acrors getting together and creating a phony management company to promote themselves through. (Well, maybe phony is the wrong word since they were managing themselves, just not anyone else and since managers aren't licensed anyhow...) I think you could perhaps make an argument that you and your group of friends were going to form a company (hell, why not just form a company) and self-manage. You could pool resources and share insight, work, etc...
BUT the idea of pretending to be an agent/agency--which would be licensed and bonded--or worse yet pretending your submission was from an existing agent--yikes!!! Apart from the point Secret Agent Man made about it being awfully weird for an agent to submit ONE actor (and besides the fact that same agent probably did submit an envelope him or herself) you are relying on the casting people NOT to know the agent/agency. If they do, which they quite possibly will--this is a small business--they will certainly call the agent directly, ignoring your personal #, and then BAM the jig is up--and they'll remember you as the person that lied about your rep.
Now, maybe I am not understanding your question....? Are you in fact asking about using an existing agency's name?

Thanks for the responses!
It was pretty much what I suspected, because the casting agency I worked for was an anomaly-- they didn't use electronic submissions AT ALL. So sometimes there WOULD be one submission from a major agency because they missed a person or two in the first mailing and couldn't just send an LA Casting link. So for that ONE instance, no one would have noticed.
I do think as a general rule, dishonesty is to be avoided, but had I been the one opening the envelope and noticed, I would have found it funny. (Considering 75% of headshots don't actually look like the actors we called in, well, that was the deception with which I was irritated...)

Then again, I'm clearly not a casting director!

mmm well, I guess if you are really signed with the agency and you put them down as your representation, then I don't think that would be a problem, BUT if you mean to use their logo without them even knowing you then I don't think is a good idea.

I worked/interned at a well respected, very busy casting office in LA---When submissions came through (hand delivered or mailed) they were treated essentially the same whether they were self submission or agents that were not part of the biggies. The exception was if they were known for niche markets of talent.