Hmmm, lets see... maybe put a weather baloon in the trunk plumbed in prior to the regulated output.... Just add a spark plug to the weather baloon for a self-destruct mechanism just in case the petrol police get after you....
On a more serious note, hydroxy storage is a dangerous proposition. That was one issue that I put a lot of thought into trying to solve. One solution could be to use another fuel, such as propane, just for times of high demand. Otherwise, you would need to seperate the gases and store the hydrogen seperately.
Bob