I must say that it surprises me a little that Yahoo in 2014 still does not provide an API to their calendar system. "A few unsupported calls" is an embarassing way to answer a question on this subject. It's good that your calendar are at least using a standard sharable format and users can subscribe to them, but that's a fairly minimal requirement in today's world where people carry all sorts of mobile devices on which a calendar is a very useful feature.
Google has even given you an opening here, because their authorisation requirements are now so arcane and badly documented that application developers will be looking for another calendar provider who makes this achievable without pulling their hair out in frustration. A single individual might be content with adding calendar information manually, but it's far more powerful to be able to create a subscribable calendar containing events of interest to a wider group.