You? Are you one of these experienced writers? One of those experienced writers who uses words like "learnt," spells "where" with no H, and is afraid of periods?
You can't learn anything about flow, delivery, intonation, or a whole slew of other things that relate to rap, as it's a performance genre of poetry. If you mean that you can learn really extremely technical things like definitions, then sure. You can learn what an alliteration is, but you won't learn how to apply it. You can learn algorithms that people use for generating more rhyming phrases, but if your issue is coming up with rhymes-- which in my opinion is the easiest part of rapping-- then you really don't need to be rapping in the first place.
So yes, you can learn some things from message boards. But the things that you can learn from message boards are things that, if you actually have any chance at being any good, you shouldn't need a message board to learn.
So just don't bother.