Craft 2.6 incorrect string value SQL error for 4 byte UTF-8 characters when saving entries
So it looks like our Craft site had an older version of the Dukt Twitter plugin installed. In a newer version it looks like the Twitter keywords have been handled better with:

return StringHelper::encodeMb4($keywords);

Which is similar to the same work done in the Craft core files for emojis.

https://github.com/craftcms/cms/issues/1627

Updating the plugin seems to have solved the issue saving on my local development environment, so not a direct answer, but a better solution than messing around with the the DB schema/collation.