I was thinking of advertising my Youtube channel that has Mapleroyal videos on it using a chalk board in game. Is this bannable?
According to the Games Terms & Conditions, as long as you aren't advertising for commercial gain, you should be fine. Usually YouTube videos don't fall under commercial (unless it's a video re-directing viewers to buy something offsite). GL with your videos! Hope to see your chalkboard in game some time.
The direct wording of the term is: As long as your videos don't advertise a "commercial opportunity" or an "investment opportunity" then you're good to go. Also the amount of money you make on YouTube vids if it ever gets big enough to hit that is like a couple cents per 100,00+ view unless you're a big name. If you're absolutely concerned, you can always turn off the option to obtain money through your YouTube videos, though I don't think it's really against any rules. You're just sharing content, and it's free to the viewers to watch. They don't need to pay YOU anything to watch it.
Even if it's not worth a ban, I think I'd be bad publicity. Can't think of anyone who would be interested to visit a YouTube channel advertised on a chalk board in-game (or using the megaphone for that reason). Do you?