I cannot find a repository for QGIS (2.18 or LTR 2.14) compatible with the latest Ubuntu distro (17.04) in any of the PPA's listed in the installer docs. Do I have to wait for a 17.04 supported release, or is there perhaps a workaround?
QGIS LTR (2.14) is in the main 17.04 repository.
The repository for QGIS latest release (http://qgis.org/debian) currently provides a QGIS 2.18 package for 16.10 but not 17.04, and the 16.10 package won't install on 17.04.
The nightlies for the upcoming point release are available for 17.04, though. Currently that's the only way to install 2.18 for Zesty from a repo. The usual cautions about installing nightly builds apply.
deb http://qgis.org/debian-nightly-release zesty main
Since 17.04 is available in the nightlies, the main repository will presumably include a 17.04 package for the next point release for 2.18.