I have thought of one before, but it never got built. I love the idea of having a whole fermenter full of beer and not worrying about head space. This is what I came up with using a regular beer keg as a fermenter and making a nylon connection to a piece of stainless cookware.
The green is the nylon piece I will have machined when I get ready to make this. It is clamped onto the keg opening via a #2 tri-clover clamp.
Here you can see how it works. I designed it mainly for blow-off wort recovery not yeast harvesting or removal.
Basically I am thinking that if I fill the fermenter and connected BU device to the top of the recovery tube, that I will have a more than full keg after primary is finished. When it is time to remove the BU device, I will just put a clean towel around the inside of the keg top and un-clamp the connection. For my system, I would then install the keg spear (3/4" cut off bottom) and reseal the keg. All the overflow, maybe a couple of cups will get cleaned up by the towel and then you can clean the top better after you are all sealed up. Just my version, so I thought I would share.