Sounds like a plan. Maybe you can code something in the forum that they only get the "DOFOLLOW" if there is more than "X" content, but have that as a random number between a set of numbers.
My concern for the forum has to do with people posting "yeah" or "I agree" and nothing else, just so they can get their signature on the thread. The thread gains Domain Authority and then they can leech some of your authority to their site as a result.
But I definitely respect your idea of leaving it as dofollow, it's just not normal to see that these days.
Fingers crossed for you. If I can be of any help, feel free to hit me up.