Most famously used in the bombardment of Fort McHenry during the War of 1812 only 5 years after the last Battle of Copenhagen that saw deployment of the Christian VII that was recently added.
I think as it fits in the timeframe of NA, it should at least be considered. I also don't think there is a balance issue. The actual performance could remain the same ( only with much higher chance for a fire ) or there could alternatively also be a reskin of the Mortar Brig called HMS Terror / HMS Meteor with higher BR to compensate for potentially higher performance / buffs in damage.
Even if only the visuals would be added at some point, I think it would make for more interesting cinematics in port battles. Adding only higher fire chance seems reaonable, given the minor effect of fire ( unless used on purpose to fireship ) and the low chance of currently implemented mortar balls actually causing fire.