Perhaps I am, but when somebody says "which you can power with a 9V radio battery", they're almost always talking about one of these, which don't carry much energy and are kind of pricey for any application that doesn't have very low current draw.

A pack made of 8xAAA or 8xAA cells, especially if rechargeable would work nicely for that application. What people usually call a 9v battery, not so much.

Though the clearance lights he was mentioning do have pretty low current draw, lower than I realized -- 60-180 mA seems typical, so an alkaline 9v battery could last 3-8 hours, possibly more if the current draw is lower at 7-9 volts than at the 12.0-13.8 volts they're designed for, possibly less if the light cuts out once the voltage drops much.

Of course, consider that a PBSF or clone is 1) likely to be far more visible, and 2) lasts around 100 hours on two AAA cells (significantly cheaper than one 9v battery, and AAA rechargeables are far more practical than the pathetic 110-150 mAh 9v rechargeables they sell) in flash mode, and I think about six hours in steady mode?