I'd start by identifying charitable groups destined to receive the instruments. This will help pitch the idea to people - especially retailers. It really does legitimize the undertaking by having a actual, known/reputable and on-going charitable cause identified as the end recipient. While I might sound cynical, this is a question I've seen asked many time to people cold-calling music stores for charitable purposes.
I'd find one in each region that GT members are keen. At the moment it seems like the western cape and gauteng?
I've a few bits that can go to a good cause (old straps, capo's, slides, books, soft bags). Can also get pretty cheap new and secondhand parts . And if someone's stuck on a setup/build I'm happy to help out - ala secondline support - I'm not one to drive the re-building process.
One place we might be able to find a lot of donations is instruments with headstock breaks, bridge issues and/or missing bracing. Unless it's a gibson/martin/taylor, there's not a lot of value in repairing them. But once you have a jig going, the repair (or so I'm told ?) is fairly straightforward. If anyone in Cpt is interested, I reckon can arrange a 'training' session with someone who knows what they are doing (e.g. Not me)
FInally, I wouldn't limit to only acoustics. There's many a weird and wonderful incomplete projects that someone will want to see be used by someone. Perhaps even do some funky builds like a broomstick bass, cheap n cheerful cajon's, etc... In time, sure we can find someone who can put together cheap n cheerful amps for the electric side of things too. (Headphone amps, or 1W transistor amps into a old car speaker).
