There are always non-technical topics presented at FIRST Championship Conferences. I have presented for many years, always on a non-technical topic, as have lots of teams/others. Marketing and branding have been presented before but don't let that stop you from submitting a proposal when the request comes out - there are always new people in FIRST who are interested in topics that have been given in the past.
I usually have 100-200 people in attendance.
In addition to reviewing previous Conference listings, look at
http://www.firstnemo.org/resources.htm - some previous conference presentations are listed there.