You might want to look up Amazix. They're a consultancy that also manages bounty campaigns and maintains a separate site listing all such projects. Same guys behind Bancor bounty campaign last year, so that's a pretty big credential to have as far as bounties go. This isn't exactly what you're looking for since it doesn't have a list of "trusted bounty campaigns" but a list of all the projects it manages. So, if you trust Amazix, then everything on their list will be suitable for you. If you can work within very strict requirements, that is.