@sibelman IMO, way too complicated to work easily. I would also mention that the humidity generated by fermentation will make the hop pellets swell up making things more difficult for you. Far easier to simply leave the sight glass empty (and the valve closed) until it's time to add the hops to the batch.
I fail to see the attraction people seem to have towards using the CO2 generated during fermentation to 'purge' the kegs that will get the beer later. I know breweries (when they get big enough) capture the CO2 produced, compress it, and then store it in bottles for use in other areas. That requires a good amount of hardware to also reduce impurities in the CO2 being harvested.
I have a recipe on tap that was placed into keg about five or six months ago. It's an English ordinary bitter. Zero issues with it. I filled from fermenter (fully carbonated). I did a purge cycle on the keg post sanitization. Filled it with the gas post feeding into a container partially filled with Starsan (to act as an airlock). Poured some out yesterday, still good.
I do understand the desire to purge as much oxygen as possible from a batch when adding dry hops. It's easy enough to do, without making it more complicated (for little, if any, benefit).
@Mr.Wyatt I found that sight glass doesn't seal all that well. Plus it doesn't hold as much as the ones with four bolts holding it together. I could only get about two ounces of hop pellets into mine. I put it aside and am using the traditional design type, which can hold at least three ounces of hops. While that sight glass is a neat idea, I think it fell a bit short during implementation.