Here is some clarification from the United States Coast Guard Auxiliary....
I am a full time guide with a Masters Captains License. I am on the water 180-250 days a year.
I have the USCG, on my boat about 10-15 times a year. My stuff is in order so they actually board me to do trainings. I see them in close proximity almost every day that i am on the water. TPWD is on my boat about 6-8 times a year. And the local sheriffs do safety inspections at the dock often.
Have not had the capacity placard on my last 4 boats over the last 12 years. 2 i removed and 2 flat out didnt have it when i bought the boat used. Out of the 100s of boardings and inspections i have never been asked for it. Because it is meaningless to any law enforcement.
Ask law enforcement yourself. Yes they require the manufacturer to put the placard at manufacturing. But there is nothing enforceable under any law on that placard. You cannot be ticketed for not having the placard. You cannot be ticketed for exceeding the placard capacities even if you have the placard. The placards are not issued by the USCG at all in any way. They are simply printed up by the manufacturer as they are required to do so. If you have graphics in your computer you can make one up yourself and have it laminated at a print shop. It will be just as legal as the one from the manufacturer.
You can absolutely put as many people as you have life jackets for. You can also put whatever horsepower you want on whatever boat you have. The placard is a CYA and nothing more. If you want to be an idiot by grossly overloading your vessel you certainly can.
If you feel the need to get one there are companies that make them. You can have it read whatever you want. There are some standard math formulas they use to calculate by but those formulas in themselves are completely arbitrary as well.
On boats less than 20 feet in length with no capacity plate, use the following rule of thumb to calculate the number of persons (weighing 150 lbs. each, on average) the vessel can carry safely in good weather conditions.
Number of people = vessel length (ft.) x vessel width (ft.) รท 15
The Coast Guard Maximum Horsepower Capacity is determined by a very simple formula for vessels less than 20 feet it is:
(length x width x 2) - 90 rounded up to the nearest 5.