I've just looked it up...

The CHEC kit is frigidaire sized. Can't be used in cars. I was partially right about metering being a problem... the actual problem is hydrogen delivery to the combustion chamber... another problem is efficiency. If you take enough electricity out of the alternator to electrolyse enough water to create enough hydrogen to make a difference in combustion, you're actually on the losing end of the economy scale. Apparently, the new systems are more effective. Plus storage of the hydrogen isn't so difficult, since they don't need a high pressure tank or fuel cell matrix. The hydrogen is stored at 22 psi... this probably accounts for part of the bulk of the device, the low storage pressure.

Another hydrogen injection concern is actually suing CHEC for copyright infringement.

Back to the drawing board. Although creating an electrolysis device for the car and a storage system for hydrogen wouldn't be difficult, the efficiency increase will likely be minimal or non-existent for a backyard set-up.