Until they can sort out the passport issue in Koln and Frankfurt I can't see this happening. Detraining everyone in Lille or Brussels for passport checks will add 75 mins to the journey and kill the time advantage.
I was at Koln Hauptbahnhof last week. I used the Thalys service, which departed for Brussels from Platform 8(D-G), so I assume a London service would do the same. FYI, platform 8 is an island platform, with (I think) platform 9.
The area underneath the platforms is currently used by a large number of shops/restaurants. It's practically its own shopping mall. It wouldn't be the hardest (or most expensive) thing in the world to convert part of this space to passport control, a waiting area and a travelator up to platform level. There is so much retail space down there that the loss of some of it wouldn't be a terrible thing. (Just don't get rid of the Bratwurst place!) Plus, the area of the mall under Pl 8 isn't the prime real estate, as it's not near the main entrance.
The 200m Thalys train only took up half the platform, so a 400m Eurostar set shouldn't have a great problem. Assuming the same solution currently being suggested for Amsterdam, a screen could be used to separate platforms 8 and 9, and the London-bound section of a Eurostar from the Schengen-bound section. My preference would be for clear barriers with plenty of doors. The doors could then be open between services, and the relevant areas only secured when needed. I assume it would be something like 1 train per 2 hours, so most of the time the platforms would be relatively open.
There is a lot of space between tracks, and (what we in the UK call) the 6 foot already has a number of structures in it - plant buildings, I think. Putting a fence between the tracks for 7 and 8 would therefore not be a big problem (IMO).
My train from Brussels arrived on platform 3 or 4 (I think - I wasn't paying too much attention). Bearing in mind there was no passport control on arrival at Brussels from London (it was all done at St P), I would imagine the same would apply at Koln for arrivals from London. Obviously, this may change in the future, though.
I can't speak for Frankfurt, having not yet been there. Personally speaking, I would love a direct train to at least Koln (more than any other city in Germany), but that's because I know someone there.
