I need to dig into this[1] a little further, but just to note that the workaround is to use the old method:
require_once "$IP/extensions/LastModified/LastModified.php";
(that is call {{ExtensionInstall}} straight up, with no |registration=1
).
What may need to be done is two sets of installation instructions: one for the Git master and the other for the downloads from the ExtensionDistributor.