Great that there is so much support regarding this!
I agree it is hard to define. The published information should be sufficient to at least understand what the team was doing and why, so that other teams can understand. In case hardware and software is developed by 3rd parties, everything should be published in a way that it is reproducible (i.e. a detailed Bill of Material for a PCB, manufacturing data for a PCB, the Software and at least some basic instructions).
I just checked, in Rescue we actually have this rule:
4.3.2 Teams awarded with certificates are required to post their documents and presentation online when asked by the OC/TC.
It was probably just not enforced. This rule mentions documents, I think software and hardware documentation are documents as well, but it is not clear.
This forum would be a great place to publish this, but a new section for sharing would be necessary. Teams that want to share their robots could do it there voluntarily or after being asked.
The question is if it is good if teams are forced to publish everything. It would be better to encourage them, but it should be the teams decision.
Actually in the Majors SPL league teams are only required to release their code without code that defines the behavior of their robot and perhaps configuration files. Otherwise all teams would use the last years winners code, since the hardware platform is the same. I see a similar problem in Junior - if a team develops a great robot based on LEGO and needs to publish detailed building instructions and all the core software, there would be many lookalike robots with identical software in the next year. I understand that this is generally desirable in a research competition, but in Junior possibly many teams would not learn as much as if they have built it on their own.
Also my original question referred to the work not done by the students. It's a difference, since what the mentors etc... provide is on the same level like LEGO, but usually does not solve core RoboCup tasks.
To summarize, I am a supporter of open source, but there are some understandable reasons why it might not be good to make the whole RoboCup Junior competition open source. Teams should have a platform to share and be encouraged to use it (like the forum). But it should be mandatory to publish work not done by the students and not already accessible for everyone else (if, for example a mentor develops a robotics platform and makes it available commercially world wide that is fine, but if the mentor makes it only for the own school, it needs to be published in a way that every other team and competition organizers can reproduce and understand the work).