For a long time I've been thinking that there really should be couchsurfing or airbnb type site but for adv motorcycle sharing. I even considered starting such a site. The problem is with trust and due diligence, and figuring out where exactly this kind of site would fit into this equation.
I also think the "bike swap" idea is almost a non-starter. The idea that 2 individuals will find each other's locations appealing and the scheduling will work for both of them - it's like caching lightening in a bottle.
I would like to report that one kind individual did rent me their motorcycle in South Africa for 6 weeks and I ended up riding around 6 countries in that region. I think what helped me is that I have some ride reports online, website, etc. I never asked, but I presumed that the person Googled around to see who I am, where I live, where I work, etc to assuage their fears and concerns.