Hi Simon,
The best way to contact the support team about being a volunteer is by filling out a ticket here.
https://khanacademy.zendesk.com/hc/en-us/requests/new?ticket_form_id=182867
From there someone can be in contact with you and get the idea directly to the team.
Thanks you for supporting Khan academy.
Should you have any questions at all I will be happy to help you.

Hi Simon, I just thought I'd add this:

Don't fail to vote up the earlier suggestion to make an IB math mission for KA! 

I'm sure your initiative would be popular with  a number of students, and it would, technically be possible to publish such a playlist in the user created programs in the CS section, using println url's not clickable links. However, if you've been using Khan Academy for some time you'll know that it truly is a dynamic site. If you do all that work without Khan Academy agreeing to making it a mission - which would probably require contact with the IB organization the way things are usually done - I fear there will be url's that don't work very soon. Please take that into account if you want to start making a mock -up of your idea.

Seeing that this is the month of May already I don't think it is likely that a decision about creating an IB playlist for Khan Academy will become a reality for this summer. I love the fact that you want to contribute in your spare time during the summer months, though, and whatever happens we can always use another person who browses through the math section and tries to answer student questions. If you are doing IB math you are quite advanced and are likely to be an asset for both middle- and high school students.

Hi Simon,
There are also other ways to volunteer with Khan Academy currently we have a Support Advocate program (early stages) you can check this article for more information
https://khanacademy.zendesk.com/hc/en-us/articles/202947814-How-can-I-help-contribute-to-the-Khan-Academy-Support-Community-

Again Thank you for your commitment and dedication to Khan Academy as you will see we are always growing so having more help in answering users questions related to mathematics is really appreciated we value your time here.

Should you have any further questions I will be happy to help you.
Proud to be a Khan Academer
Katrina

I am a teacher and have just transferred to Malaysia from Egypt.  I am an American and have used Khan Academy with great success for years.  

I am not teaching in an IB school and need missions for the Diploma Program Math Studies and would appreciate any information you might have about the status of this being done or ideas about using the program to automate my job of teaching math.

Thanks again and look forward to hearing from someone.

Max