[chrishajer]

I think you could do this with a theme switching plugin:

https://bbpress.org/plugins/topic/bbpress-theme-switcher/

That particular plugin worked only through 0.9 though.

I think the idea would be to link to the forum using a direct link, specifying the template, or maybe set a cookie depending on where the visitor came from, then choosing the template based on that cookie.

I don’t think you need to modify bbPress at all – you just need to identify where the user comes from (cookie?) and set their template appropriately.