+1 to the above.
With a 20L barrel, you won't need much time compared to a full-sized (225L) barrel. Finish your primary fermentation as usual, and then transfer to the barrel. If the barrel still smells freshly of cachaça and is still wet inside, you shouldn't need to do a hot water rinse, and doing so would strip some of the flavor out of the barrel. After you're satisfied with the taste of your beer, go ahead and package it in the same way you would normally for any other beer.
In my experience, if you want to get multiple uses out of the barrel, the best plan is to have another beer ready to go into the barrel at the same time you're going to take the first beer out. This should prevent any funky bugs or bacteria from getting a foothold and you shouldn't need to sanitize in-between batches. There are options for longer-term barrel storage solutions, but I personally think the best option is to just keep it full of beer. Just remember, a full barrel is a happy barrel.
If you're an AHA member, there is a presentation from 2013 on alternative wood aging techniques that's very interesting and may help answer some of your questions.