As long as it doesn't encourage spam or intentional posting on the thread like sending PMs or emails, it's fine. In light of efialtis' ban, mprep mentioned that:
If you accepted entries to your giveaway (that doesn't have a requirement to post in one or more threads) via PM or via off-forum means (Google Forms, email, other external websites), yes, that'd be fine.
On Games and Rounds, that's totally acceptable. This thread has been doing it for 8 years:
Also if you think this promotion giveaway is good then come back daily and post on this thread for an additional 0.001 mBTC to your account.
On a side note, when I was still in that campaign some time ago, I did post there a few times. Never had my balance refilled. Still gets bumps to this day :p