There is nothing that precludes public contributions to the project. The important part is that the change be useful to 'webscale' type applications.

Prior to this announcement, the only people involved have been these companies to get it bootstrapped, so the work so far looks a bit slanted to these companies.

Taken from the FAQ:

http://webscalesql.org/faq.html

"We’ll be doing everything in the open, so everyone in the MySQL community will be able to take what they want from what we do and contribute however they like."