I think this is a good start, but I don't see how it reduces oxygen pickup vs. just dumping hops into the keg.
The container is not under pressure, so its less of a 'purge' and more of a blow out with CO2. You will absolutely reduce some O2 in the container, but considerably less than with a pressurized purge.
With CO2 purges, you are mixing the CO2 with air, diluting the O2 concentration. In a keg, hop cannon, or other pressure vessel, you can increase the vessel pressure with CO2, proportionally decreasing the concentration of O2. In a non-pressurized container, you're pretty much relying on displacement by gas flow through it, meaning you're using a LOT of CO2 to mechanically displace some
air. Because its not sealed, oxygen quickly flows back in after air flow stops to regain equilibrium inside and outside of the container.
I also think you might inadvertently blow a bunch of hop dust everywhere.
You might have better luck displacing O2 with a heavy inert gas like Argon. Since your SiL is a welder, he might be able to help you out here
. Still, the inversion of the unsealed container will inevitably force the heavy gas out and the light gas in.
Better still is to add the hops to a purged and empty keg, purge it a few times after adding hops, then transfer the beer on top from another keg. I think this method tops all others in terms of O2 reduction, including the 'hop cannon' used by several pro breweries. Pro brewers can't do it this way because they need hops to distribute throughout their tall vessels. Our surface area/volume ratio is much larger, and if you want more you can just tip the keg over.
If you want to keep plugging away with the mini-hop-cannon project, I'm sure we could think of some ways to install it into a closed, purge-able location. We'd have to call it a 'hop-sparkler' or something, though. (Trademark/patent pending)
Sorry for the long-winded answer and half-science babble. I've had some free time at work lately