I have a very hard time seeing this as anywhere plausible. Ships of the Line have this unique issue where as they are vastly underpowered as of current, but don't need much tweaking to be absolutely broken. Only sporting the broadside count of a War Galleon as
@Arr mentioned in their post above, they are huge bricks in the water that would be vastly slower than ALL adversaries, primarily just sporting an additional Health Pool and armor.
This presents the problem, that if we buff the speed of a SOTL, it no longer is slow and cumbersome, and in stead turns can achieve 15 knots naturally and turns on a dime. Buffing the broadside count of an SOTL proves the issue that they would have advantages against all currently existing warships, and keeping it the same would prove that it's presently weaker than a War Galleon overall.
Finally, buffing the Health/Armor will make it impossible to kill in SvS, and wouldn't change its relevant playstyle in SvE, as any ships that garner full explosive broadsides such as the HMS Battle Royale, EITC Corsair, Spanish Cerberus, etc, would still very much be able to one shot it very consistently due to its massive hitbox.
TL;DR, In short, the ship being available to pirate players would cause extreme balance issues, and there are no easy ways to get around this with a ship that was designed to be inevitably top tier. SOTL will likely not see release as a pirate ship, but in other variants down the road possibly.
This is a fantastic idea, and I could see this being a good way to add interaction to the Ship-of-the-Line for treasure fleets and things of that sort, to add an extra bit of challenge.