Thank you to my fellow forum members for the compliments!
A hydrogen explosion would certainly be entertaining (makes me think of the line from Mythbusters, "Jamie wants BIG BOOM!").
I don't think a laser would ignite the hydrogen straight from a balloon though. The laser would need to ignite something else (like flash paper) attached to the balloon in order for the hydrogen to ignite. Without a solid ignition source the laser would almost certainly just pass through the hydrogen gas after bursting the balloon and it would escape harmlessly into the air.
What would work would be to perform the same demonstration I performed but just inflate the balloon with hydrogen instead of air. A much smaller bit of flash paper (or anything that the laser could burn and/or ignite really) would all that would be needed.
Small diameter balloon and outdoors where nobody can see please. Just in case, because you never know. Theoretically like you said, it should escape harmlessly, but I'm not completely persuaded that there would be no oxygen molecules trapped in the flash paper.
Edit: I'll admit that the someone who got their eyebrows singed was me. LOL It was a long time ago when I had to take a chem class. The balloon diameter might even have been 18 inches. It sounded like a sonic boom and was done in the back yard. Neighbors came running out of their houses. I was sure someone was going to call the police, but luckily it caused no damage other than my ringing ears and no police. ;-)
You can do a 2:1 mixture of hydrogen and oxygen by volume, that results in pretty big bangs... something totally different from just filling a balloon with hydrogen gas that burns with air once its ignited.
I'd recommend extreme caution though, with a volume of a couple of liters, you will need ear and eye protection. As a chemistry demo i ignited testtubes filled with this mixture, and those already produce a somewhat scary bang. They dont shatter if you ignite the mixture from the open end, but if you would put in a cork and then ignite the gas somehow, i think they might.
Anyway, I know I helped diverge this thread but it's probably a good time to stop before it gets deleted. Netscott you have me wondering now whether that experiment could be used to demonstrate the theory behind a hot air balloon? Not sure if the theory is altered in any way by having the balloon sealed but would be awesome if you generated enough heat inside the balloon to make the air less dense than the air outside (might take a fair bit since the balloon started with more dense air than outside) and made it lift off the desk. Might not be possible due to something i'm not thinking about.
Such a demonstration could be done safely. If you take the original idea of igniting flash paper inside a balloon, that would pretty much do it. It might be possible to construct some sort of open-ended balloon stucture that lifts off with the hot gasses inside it.