Wow quick reply.. I actually was just about to post that I was trying this while logged in as a manager, not realizing that there was and admin role. It got much further as admin, but still has some failures:
Check packager version 0.1.0
Plugin name Plugin Manager
Plugin version 0.1.0
Check the Apache version No test performed/required
Check the PHP version OK
Check the SQL version No test performed/required
Check the Mantis version OK
Check the Plugins version OK
Install or Upgrade? Install
Check install script OK
Install the plugin
Step 1 / 3: SQL upgrade OK
Install the plugin
Step 2 / 3: Patching files FAIL
Install the plugin
Step 1 / 2: Patching files FAIL
Install the plugin
Step 2 / 2: SQL upgrade OK
Restore old files OK
Installation FAIL
I probably missed something else obvious, or trying to install as a manager broke something, but any help would be greatly appreciated.