Ok, that works. It stops the plugin's action from showing up on the list for user's to pick when the dropdown is non-device, but if its already on the toolbar, it stays there. This is good enough. Curious as to why it crashes, but with the workaround its a non-issue. Thanks.
trivial FYI - while testing add/removing the plugin, I noticed if you remove a plugin that does not exist there is a missing space in the output:
$ calibre-customize -r 'No such plugin'
No custom pluginnamed No such plugin