Hi! Maybe this is a misunderstanding due to FOSDEM lingo - call for participation is not an registration/invite, it is the equivalent of call for papers (that is, talk submissions).
FOSDEM [1] is open to all (there is no registration or fees) - and several thousands of people come every year.
But otherwise it is a just show up event? Also if it is too late to give a talk, could one do a poster etc?
We don't do poster sessions, but we have a dedicated slot for lightning talks in one of the two days. Shoot us an email at info@qosf.org and we'll see if it's a good fit.