I thought of that, and it is certainly a good solution as well. But in terms of ease of programming for their installer, it would be simpler to just change "release" to "snapshot" than to determine the newest version in the launcher, read its JSON file, find the release date, parse it back into a value, then decrement it. (EDIT: Nevermind, I guess they could just obtain the release time and hard-code their own value lesser than that.) It would also allow them to keep the actual dates of the Forge releases in the JSON, which will naturally always be after the vanilla release.
It's a shame that the launcher barfs when you try to use something other than "snapshot" or "release" to classify an entry.