Hmm. That's a interesting question. I'd like the games themselves to be open source, but your own code base doesn't need to be released. We would just need some way to confirm that you are using SFML if it is hidden.
Modifications might be fine as long as they are started, depending on what they are I suppose. Either that, or they shouldn't be allowed at all. We're trying to showcase vanilla SFML after all.
May I ask what parts of SFML you are modifying?
Minor tweaks to stability in networking code regarding sockets in SFML 1.6.
My solution isn't appropriate to SFML (I spoke with Laurent before so he knows the score) but it's mandatory to ensure stability when connecting and polling websites, more specifically,
non-blocking sockets don't connect, and blocking sockets freeze if there's no connection in 1.6.
I don't know what Laurent has done for 2.0 or 2.1, but I had to work in a solution that gives the non-blocking sockets a poll-delay so they can connect, so my version of SFML is slightly modified. It's not fundamentally different from SFML overall but it wouldn't function stably on default 1.6 DLLs.