I have seen initiatives like that in the past but they are often dogged with problems (fake sign ups, spam posts etc)
Instead, I would advise you give people a reason to join and/or participate in the forum - such as with giveaways or competitions.
We had one on one of my forums where every month for an entire year we gave away a Black & Decker Steam mop - I think it worked well. Members needed to log in at least once a week to get their weekly entry - but we threw in bonus entries several times a month too, which gave people a reason to log in every day.