Sorry for the late response (was away last week and missed your post from 3 weeks ago). What is the value you have for the page.head_value_separated.title text key?
I reviewed the code again and it looks like the ForumTopicPage sets an override that uses the message topic instead of the board name. Sorry for the confusion.
So back to my original question given the tools available to us are we able to add the board name?
It looks like there is currently no way to get the board name into the <title> tag when the ForumTopicPage loads. You could change the <title> tag after the page loads, via javascript, but that wouldn't help with SEO since most crawlers just take what the <title> is when the page loads.
