Is is possible for one to selectively install ebuilds from the Funtoo Experimental branch without having to switch entirely from the Funtoo Current or Funtoo Stable branches?
Yes it is, either by unmasking the package or by copying it into a local overlay and unmask it in the package ebuild then.
Please consult the Gentoo Handbook for these kind of questions first: Gentoo Handbook Part 3, Chapter 3.2
Even when it is the topic of how to use an EOL kernel, this Blog post describes how to do the later variant generally, using EOL kernel as an example: Use an EOL Kernel
I ask this question in a context similar to that EOL Kernel blog; where the version/ebuild is non-existent rather than simply masked. My example is sys-libs/glibc-2.17 which is currently only available under the Funtoo Experimental branch.
Mixing the experimental branch with current or stable is not that straightforward : those are different git repositories. Commits (changes) in experimental might not be available in current / stable (even hard-masked).
For this particular issue, glibc is now updated in current (hard-masked), and the experimental tree will now be in sync with current for a while.
However for the record, a work-around for a similar question in the future would be to clone experimental-mini-2011 git repo somewhere else, create a local overlay, and copy or symlink the ebuilds that you want from the clone to your overlay.
Remember howerver that any ebuild from the experimental tree might break your Funtoo, it's called experimental for a reason :-)