Ok. Another idea: It could easily be a subscription option “auto-Or, how about a Follow button next to the Like button in the bottom bar under the message. Perhaps only showing when your Subscription is Following Only, otherwise in the More menu.
As well as in the More menu when in List view (Topics or Messages).
The text of the menu item, as well as the tool-tip of the button, should make it clear that the function is /Follow Topic/ - the same as using the link in an emailed message.
I've always believed that the Follow and Mute functionality deserves exposure in the groups' Messages pages, and not be hidden away just in that compendium place that I never visit.