Looking at it from a different angle have you thought of approaching a leading supermarket and ask them if they would allow you to have a table laid out and a small display about the charity you could also attempt to recruit helpers and staff at this event also.
Failing that, at least you'll of collected a bit for your charity and make your self more known to the local community. You'll get to meet a wide range of people from all walks of life and indeed a few retired folk who may be interested in your admin job.
But unless you get out there and sell yourself no ones ever going to know