Sorry to know that visa rejection news. As my thought visa process fully depends on the embassy. It's not guaranteed any chapter even WMF to ensure that If they provide invitation letter that means anyone got a visa!
I may share my last Wikimania Mexico experience at this point. Mexico Embassy not available in Bangladesh and If we need Mexico visa we need to visit India. But It's not good for me to go India for a visa so that I follow different ways. I informed that If I have a valid USA visa then I may enter Mexico. So I go to USA embassy and I submit all documents provided by Wikimedia Mexico. I am lucky enough I got USA visa and attend Wikimania Mexico. In the same time, one of my fellow Wikipedian go to India for Mexico visa and got rejected. After back I suggest him to apply for the USA via using same documents and submits. And finally, we both got USA visa and attend Wikimania Mexico.
What exactly I point that, In my many International conferences attend experience I see, Visa processing fully depend on the particular embassy. Its totally depend on them if they provide visa or not. But the documents may help to get visa easily.
Hasive