Exposing a new group of people (i.e. the coffee house/pub/music school) is a good idea. More people see, more people want. Two considerations. First, many hands will be handling said instruments with questionable cleanliness and/or respect to the fact that the instrument is intended to incite education and potentially, a sale or two. Any instruments offered for "learning/promo" can likely be written off as a donation.
Second, to the topic of "consignment" relative to my own experience. Before I found a proper way to sell my craft in building cbgs, I tried "consignment". I had net yet achieved any form of confidence in way I had built, and was too eager to bend over backwards just to make a sale. The average percentage that businesses wanted for consignment was 35%. When factoring in the cost of parts and my time, giving a stranger 35% of my sale was painful.
Others may have had a different experience with consignment. My own opinion is that consignment is not a good choice. Exposing new people to cbgs IS a good choice. Best of luck.
Another option you could consider would be wholesale. Just think what would your bottom dollar be that you could sell your CBGs at and you still make the money you want (provided they buy several (say 3-5+)).
Basically you could "cut him a deal" if he bought 3-5 (or more) that way you are able to make money and he is able to then upcharge, and sell the guitars to students at a rate he could also make money on. You could also charge more a piece if he only buys 3, and charge slightly less a piece if he buys 5 or more.
I know this may sound complicated, but typically for most things your wholesale rate might be:
parts + time x 2 = $
Some people would rather buy outright, and then charge what they want, then have to keep up with consignment.
From a shop keepers perspective, (which I have been), I would much rather do consignment at first moving to wholesale after it becomes a proven seller. Consignment is risky for me because I am responsible for the item prior to sale but, I take a lot of burden off the guitar maker. For that, i get a decent cut of the sale.
Here's another idea I had rolling around in my head. (speaking of demos)
Lend or give one to a friend who plays in a very consistently working band, he plays slide very well and the band has (locally) a large following. They use social media and work hard to keep the fans coming to shows.