The challenges that I see: 1) you will be fermenting on the entirety of the trub. Some trub is fine, but all of it? Not sure how that final beer would taste. 2) Given that all the spent hops will be in the fermenter, you better hope that outlet doesn't clog. 3) There will be a CO2 blanket only while CO2 is being created. Once fermentation ends and that positive pressure goes away, you will get oxygen ingress, as you are not in a sealed fermenter. You'd want to monitor that gravity closely and transfer as soon as it's stable.
But overall I see no reason why this wouldn't work well.