Uploading to Extras-devel will have its library dependencies listed as the lowest which meets its requirements. If that includes PR1.2 APIs, then it indeed won't be installable by PR1.1 users. However, if it's a new package - rather than a new version to an existing package - that won't cause problems to anyone.

So, fbreader-experimental would be an obvious name.

What PR1.2 symbols does 0.14 require?