Here is the DMV regulations - It clearly states that the CA registration numbers need to be fixed to the vessel preferably at the bow on a non-removable portion of the hull so this would rule out putting it on the boom.
I even had one officer at Santa Cruz Harbor years ago give me a hard time for there was not the correct spacing between the CF numbers and the CF numbers and the registration sticker.
Link - ABCs of the California Boating Law
http://dbw.ca.gov/Pubs/Abc/registration.pdf"Display of Numbers and Stickers"
"Numbers and stickers issued at the time of registration must be placed on each side of the forward half of the vessel, usually on the bow, in the manner indicated below. (See diagram at web link) If placement of a number on a flared bow would result in difficult reading, the number should be placed on some other part of the forward half of the vessel where it can be easily read.
For personal watercraft, the numbers and stickers must also be affixed on each side of the forward half of the vessel on a non-removable portion of the hull. On inflatable boats or vessels so configured that a number will not properly adhere or cannot be clearly seen, the number should be painted on or attached to a backing plate along with the registration sticker. The registration stickers must be securely affixed 3 in. aft of, and in line with, the registration numbers. The number must be visible from each side of the vessel. No other numbers, letters, or devices may be placed in the vicinity of the state-assigned number.
To separate the numerals from the letters, spaces the width of the letter “C” or hyphens may be used between the prefix and the number, and between the number and the suffix. Letters and numerals must be at least three inches high, of block character, and of a color which will form a good contrast with the color of the hull or backing plate. In determining height and contrast, any border, trim, outlining, or shading around the number shall not be considered." CF 5576 JS [ ]