Take a look at how BuddyPress Activity Plus appends shortcodes to the end of activity posts. Then, have a look at BuddyPress Activity Stream Hashtags.
The concept would be to append shortcodes or a hashtag on form submission. The hashtag would be interpreted by the activity stream hashtags OR the shortcode would be interpreted during the activity stream loop.
The difference is pre-processing the data to become links or post-processing data when the page loads.
I’m still working on something similar. I need to have it done by tomorrow, so I’ve been a little sloppy about it.
1. Edit activity/post-form.php (modify the form to add your new fields)
2. Add JS to use your selections to insert hashtag or shortcode to the end of the activity textarea.
3. Add filtering links to show hashtag filter OR write custom filtering system