I have no great wish to see ranks inflated on a popularity vote but the role of 'ambassador' is, I am sure you would agree, better served in a senior position rather than in that of a junior officer no matter their previous credentials!
In which case give the role to someone with relevant experience and credibility from within the service. There are plenty of ACMs, AMs and AVMs happy to serve as Fg Offs on AEFs, I'm sure you could find one happy to step down to Gp Capt. However, if you are going to effectively turn the role in to a marketing position by giving it to somebody on the basis of their media access and credentials, it doesn't need to come with a commission regardless of who they are or how well they fill the uniform.