I didn't go to the screening, but I have some interesting information.
As we already knew the 70mm print came from the BFI. Cinema Ritrovato stated that it was a 1990 print with Kubrick-approved color grading. I speaked with the projectionist and he told me that it was in a Dolby Baby Boom format (tracks 2 & 4 for subwoofers). He said that the sound was "Dolby SR 42 format", as this picture confirms:

https://www.facebook.com/CinetecaBologna/photos/ms.c.eJxlzMENACAIQ9GNDKWAsP9ixoMH4frSfgicYeWUUIK68EQrJT~_heHUB2IWjY~_M1hNk7tvvG4soBlWcjzg~-~-.bps./10153649529808132/?type=1&theater

However, according to this site, the 42 format is A-type, while the 45 is SR:

http://www.in70mm.com/newsletter/2000/62/dolby/

So probably there was an error on the label: it is either format 42 (hence A-type, not SR), or SR (hence format 45, not 42).

The BFI archive entry for 2001 doesn't say much about the soundtrack:

http://collections-search.bfi.org.uk/web/Details/ChoiceFilmItems/153451583

Anyway I was surprised that a Kubrick-approved print exists with a Baby Boom format, which didn't exist in 1968. So the question is: were the original left-center and right-center channels just a duplicate of the left and right channels?

According to this article, when the film was restored in 2001 they decided to use Dolby SR but to stay faithful to the original 6-track format. No mention is made to a Baby Boom version.

http://www.arcadiacinema.com/melzo/rassegnastampa/200109_cinematechnology.pdf

What do you think? I'll try and ask information to the BFI, they were very kind to me in the past.

P.S.: here are some other pictures of the screening:

https://www.facebook.com/media/set/?set=a.10153653575613132.1073742255.355192863131&type=3