Is it possible to use "Flash channels" to send non-transactional data, A.K.A 'zero-value' transaction type which simply broadcasts a message or data?
If so, is it free? And how to do it?
Transactions made in flash channels are not written to the Tangle.
There are only two events that are stored:
When opening a flash channel, two (or more) entities, let's call them Alice and Bob, lock up Iotas.
This is why it wouldn't make sense to "broadcast a message or data" via Flash Channels. It would be just as good as broadcasting it without flash channels.