Perhaps it's in your best interest, Dan, to forward your request to
@Ira and let him...y'know, the Communications Director for the IBA, to weigh in on your request -
http://www.ironbuttrally.com/about/adjutants.cfm
From prior discussions held elsewhere, the actual number of members isn't
exactly 75,000+. I will allow those in the know to disclose that methodology in this thread if they choose to do so.
As far as what
@bobthekelpy mentioned above, a better representative view might be the entirety of the Iron Butt Rally statistics:
https://www.ironbutt.org/ibrfacts.htm
Finally, as far as this statement "Today I believe more than 60% of the IBA riders ride Harley-Davidson motorcycles." While that may or may not be true, my suspicion is that for the basic SS1000, I'm not convinced that's accurate. The Bar-and-Shield brand probably has a significant percentage, but I would not expect it to be a majority (greater than 50%...) of those completing certification rides.
Luck to you on your research...and my letter of support will be sent shortly. Thanks for letting us know!