This is a good question, to date the packages I make just use the version of R I happened to have had installed at the time of first development, and I leave it at that over future versions until I run into the very problem you describe where a dependency can't install due to it needing an R version that is greater.
The rhub and Travis checks are mostly concerned with the current version of R and future, but a functionality that lets you find the minimum version of R you can rely on would be super.