That's an interesting question/suggestion.

There isn't currently any built-in way to achieve precisely what you are after, but there might be a workaround that could at help you achieve a similar result.

From the Advanced Search page, you could enter a keyword of "*a", search entire post, select the forums you want to search, then select to find posts from your last visit and newer. The * works as a wildcard and placing the search string in quotes allows you to search on fewer than four characters. So, this search would find all posts containing any character followed by the letter "a" and would display the threads that match the search results in the timeframe selected.

You might have to play around with this a little (e.g., search from yesterday, try "a*", etc), but it will at the very least allow you to see most of the unread threads in the forums you are most interested in.

Again, not perfect, but I hope you find this useful.