Ok, so more follow-up...
Based on other posts I found on the web, I removed the reference to o.e.e.executable from my 'internal' location and added the following to my target platform:
<location includeAllPlatforms="false" includeConfigurePhase="true" includeMode="slicer" includeSource="true" type="InstallableUnit">
<unit id="org.eclipse.equinox.executable.feature.group" version="0.0.0"/>
When building with this configuration, everything works as expected! However, I would still prefer to 'host' the o.e.e.executable feature in my own P2 repo for various reasons (version control, single URL for config, etc).
So, the question becomes (and it might very well be a P2 thing) - why is Tycho unhappy with "my" o.e.e.executable feature. As stated previously, I don't see any errors in the logs. When inspecting my Maven P2 repository, all downloaded artifacts are the same EXCEPT for the fact that .m2/repository/p2/org/eclipse/update/feature/ contains a org.eclipse.equinox.executable directory when I am using the additional Eclipse location.
Any theories/insight appreciated!
Andy