I have started working on something similar. I have 3-4 ideas of apps I would like to do. I figure it would be far easier and way cheaper to launch landing pages for each, with mocked up "screenshots" of the apps. I plan to include on each of the apps a form to provide your email address in exchange for an invite. Then, I would do some simple SEO and word-of-mouth marketing and see what kind of response rates I would get back through this form. I figure in the end, this would help identify which application has the lowest barrier to adoption (e.g. easier discoverability, more user interest).