This is straight out of part 103-7:
19 MAXIMUM FUEL CAPACITY OF A POWERED ULTRALIGHT VEHICLE.
The maximum fuel capacity for a powered ultralight vehicle is 5 U.S. gallons. Any powered
ultralight with fuel tank(s) exceeding this capacity is ineligible for operation
as an ultralight vehicle.
a. Determination of Fuel Capacity. The total volume, including all
available space for usable and unusable fuel in the fuel tank or tanks on the
vehicle is the total fuel capacity. The fuel in the lines, pump, strainer, and
carburetor is not considered in a calculation of total volume.
It mentions nothing about fuel type, type of fuel tank or any exemptions. It just says that any tank that holds fuel can not be bigger than 5 U.S. gallons. Hydrogen is a fuel and the pressurized tank is also a fuel tank, so it would be illegal to go bigger than 5 gallons even with hydrogen.
You must realize that these rules where written in 1984 and they never expected ultralights to use a fuel such as hydrogen for power. The FAA has made amendments to part 103-7 many times in the past to adjust for changes in the sport, so there is a chance they may change the fuel volume limit to a weight limit if hydrogen becomes popular.
I am interested to see what they do with their solid oxide fuel cell. They make things a lot simpler and improve fuel cell efficiency, but they can´t use the oxygen from the surrounding air which means the fuel cell only lasts a few tanks or you need to constantly replace the oxidizer.