The scale of production is the critical factor here. If you want to make some maple syrup for your family, a pan on top of the
RMH barrel might work. Whether you will want to run your RMH for the time needed to evaporate is another question. (A rule of thumb is 1 gallon per hour per square foot of pan surface.) Humidity from 5 or 10 gallons of
water being evaporated inside your house in a day might be an issue.
If you want to do any scale of commercial production, you need an outdoor evaporator, not an RMH.