Good idea...
Think about charging a 'base storage' fee in order to cover utilities costs, stocking the fridge with 'refreshments', etc. Basically just a low C.Y.A. expenses charge based on the size of the person's project, that prevents someone just dropping something on your doorstep and not doing anything with it and making you feel used and resentful.
Find out what storage fees are in your area per month, and charge 10 percent more. I know I'd gladly pay that to have access to a well-equipped shop space to work out of.
Get enough ppl doing this and it's easy enough to sit back and use the fees as a second income, and rightly so.
Of course if ppl come by to help others with labor, tool usage that you do not provide, etc. then give them a fee reduction option as well.
I would shoot for purchasing some small, discrete cameras to set up a CC-Tv system, with a VCR with tape loop such as convenience stores use. 7 tapes will cover a weeks worth of wrenching time, and the cameras will discourage tool theft, arguments about who was doing what, etc. Another C.Y.A . measure. I have known some shady types who will not hesitate to lift whatever tools are convenient, and then claim later, "I forgot I had it in my pocket", or "I left it in the pickup bed..." If it comes to that the tapes are great evidence for the police to use, and your facilities will be known as a secure place to store projects, even if the door hangs wide open most days.
Insurance/liability is another issue, perhabs the 'good ol' boy' mentality can be a double edged sword. Buddies aside, someone getting hurt can and does happen, and I don't believe in this case simply signing a waiver to work at one's own risk would stand up in court. Something to think about. On the other hand there are times I don't do things that normally would not faze me, simply because it MAY be dangerous and I want someone around to call for help, if something bad DOES happen. Such as, I had need to get under the Kustomsledd the other day, looking for a small trans leak. With the air suspension all the way up I got it jacked up and on jackstands. I was still hesitant to get under it without someone else being around just in case it might(however small) have slipped or fallen off, because it sits **on the ground** with the air suspension down. Better to have someone else around able to call 911 just in the off chance.
Charging a bit above the normal storage fee will help prevent simple vehicle storage problems, unless you have room for that as well and can make a buck storing seasonal use items like snowmobiles, bikes, large mowers, etc. You can even do simple maintenance items like greasing, charging batteries monthly, etc and ppl may be willing to pay someone a bit more to have peace of mind, instead of merely picking up a machine from a different storage facility and facing a dead battery or flat tire, etc.
Anyhow just a few other things to consider...
IMO- there should be more ppl like you willing to do this. I remember doing the 472 swap into the 'sledd in the dead of winter with the motor/trans hanging from scaffolding, on a sloping gravel driveway... and I bet I'm not the only one who has done this...
Doc