It seems like this can be handled rather well by simply adding a 'XML-RPC access', a 'SOAP access' and a 'WebDAV access' set of permissions. we already have a 'FTP access' permission which works fine.

Not quite..... That permission controls several methods that are particularly useful for FTP (indeed, you cant do FTP without them). However you could also call those methods using xml-rpc, soap, or ZPublisher (or, not, depending on the whether you have the 'FTP access' permission). Your suggestion makes sense for WebDAV which, like FTP, only uses a small set of methods. I dont think its appropriate for a more flexible protocol like xml-rpc, soap, or ZPublisher.