Auber EZBoil

[pob]

Would this work, still keeping the BIAB bag as a filter on the inside?
Bayou Classic 44 Qt. Stainless Replacement Basket ~ turkey fryer / steam basket

13.5"d x 12.75"h should fit the keggle - as usual with US products, great value until you try & ship them.

[BrewDorg]

My tun is 28.5cm diameter and 47cm in height. I calculated that an inner pot would need >13L volume to hold a decent sized grain bill. In order to fit the pot into my narrow tun, I'm looking at a pot with ~23cm radius and ~32cm height. A pot with that narrow with that sort of height just doesn't seem to exist 

For a keggle though, those baskets look great... until shipping.

edit. Just as I post... the website you linked has this. Probably a tad too wide though @ 11" = 27.9cm

[molc]

Just under €50 with shipping using ebay guaranteed pricing:
http://www.ebay.ie/itm/Bayou-Classic-B300-Perforated-Steam-Boil-Fry-Accessory-Basket-Fits-30-Quart-/331873856128?hash=item4d45392a80:g:80wAAOSwMNxXVDC-

You could be onto something there. Would need to do some solid measurements and calculations first though to figure out how much space 1kg of grain takes up to calculate capacity. Also since there are holes in the dies, you'll have your mash water possibly not going through the grain as preferential flow will go by the path of least resistance.

What you want is something of those dimensions, with solid sides and a mesh/ holes in the bottom, then pump the liquid down through the top or up from the bottom. Also you will need to build feet/stand for the BK to keep the pot clear of the elements, as that much weight on them will eventually break seals.

[BrewDorg]

Did those calculations myself a while back. I found 1kg grain = 2.2L volume.