Alright, so last night I said I'd get the site compatible with Tapatalk and I followed through. It's weird, but in the app you have to search for the forum with the url, so just search bunretsu.proboards.com and that'll bring it up. You can login to the forum from inside the app, which will be a separate login than the one for the app itself.
At any rate, it's up and ready to use for mobile now. Maybe at some point I'll look into developing a standalone app specifically for the forum, but that costs time and money, so we'll see.