Please excuse my verbosity, but I was going to post about this on the closed ferment thread shortly, so this seems better instead.
From my experience using my 3 gallon corny as a cropper, this is what I think:
First- You are going to be cleaning the hell out of that filter housing, too, if you use it for month long contact with the beer. My 3-A Sanitary PVC tubing that I have in between my sanke and my corny is greenish hue after one brew and nothing, not even overnight PBW nor even steam, gets it out. I personally would limit any plastic involved in primary. Transfer is OK. I use one of those filter housings for water filter and beer filter.
Second- If you are like me and are trying to ferment as much volume as possible in your sanke without tons of loss (but a little is fine) then you probably will risk gunking up the valve anyways just cause you underestimate the volume of krausen. I can definitely say that I'm not going to get any close to 3 gallons of blowoff, but getting it right at 32 oz. (or whatever that thing is).... If your intention is to actually crop yeast and get enough to make it all worth it, I think you'll have a hard time getting the final fill volume exactly what you need it to be to collect enough and not over fill into the valve.
For this reason, I am going to get a few custom length dip tubes made for my sanke gas out (which I've mentioned elsewhere has corny posts on the top cap). This way if my beer level is 6" down from the top, I get out the 5" long one. Right now, its hard for me to predict top cropping volumes vs beer volume without something like that. If I have a dip that reaches 'down' to just above the beer, it won't matter if my krausen reaches to the top like it does now.
Third- I dunno if this matters to you, but the whole intent for me is to get the full selection of yeast in the same container, so that's why I top crop and then bottom crop into the same container. One thing to clean.
Fourth- Here is my major issue so far with mine and your idea also: If I am using the corny in line to do pressure ferments, like we talked about in the other thread, I have to depressurize everything in order to disconnect and connect transfer lines. Isn't a rapid depressurization going to make most of the yeast unviable? I did hear that somewhere.... (errr, lets say Jamil?)
I don't know how to get around that problem. I don't want a poppit valve in the way of anything blow-off related cause of ID mainly. But that also makes me have to depressurize not only the yeast cropper but also the primary as well. My guess is that I should do all of that before cold crashing,- even hook up my transfer lines, etc.
And Fifth- Regarding a randall (and I think the best option for all of the above):
Before I got my corny, I was considering a "stainless pressure vessel" which is just like a corny but has 4 npt ports in stead of posts. That with two beverage lines, one shorter than the other - with a stainless scrubby around the shorter one. This would also make for a good starter vessel. For me, I am trying to figure out how to do a 'decant in place' sort of thing for my 3 gallon. I grow my starters in them and then just push it into the primary. I'd like to have 2 beverage posts with one being like 4" above the bottom, so I can crash the corny and decant most of the starter wort before shaking up and push inoculaing. I will have to do something else, though with only one other port besides gas in.
Unfortunately, "stainless pressure vessels" are pretty pricy in the hundreds.
I'm also OCD about dual use.!
The mind boggles.
