Is it possible to auto append a tag/hashtag to a Chatter feed item via a trigger? If so, is the syntax simply to wrap the hashtag in square brackets immediately following the hashtag? I've noticed that when pasting @mentions and hashtags that the Publisher does not create the link, only when typing the @ symbol or # symbol does the Publisher then start the autocomplete sequence. Pasted text is usually unlinked, so I didn't know if this was even possible from a trigger.
Yes, you can use the 'new' Connect in Apex (ConnectApi.HashtagSegment), which allows you to post mentions, hashtags, et all, which were only possible via the REST API earlier.
I don't know what the exact syntax is for doing hashtags and mentions, but there's no reason why you shouldn't be able to modify the post content in a trigger.
Appending a hashtag would presumably be as simple as
post.body += '#thetag' (but with the right fields and objects ;)). To get them as links there must be more markup involved, I'd look at the content of a feed item that has such a link, and then mimic that in your trigger.