How good is the idea, that instead of hiring these tools year after year, the WMF/Wikimania grabs a set of, say 10 portable, yet good video cameras, and reuse them from Wikimania's to Wikimania?

Unless Wikimania is coming to an end in a couple of years time, I see grabbing such gadgets (if not already available) and making use of them from time to time a valid and profitable long-term investment.

Talk of volunteers? I think when a call is put up for volunteer cameramen, we can get them to before every wikimania. 

Victor brought a great video camera (property of wmf) last year to wikimania. I enjoyed using it, and if there're enough for each session, getting cameramen shouldn't be an issue.

The wikimedia community is big, thus aside maybe needing the expert/professional services of  production houses to handle live streaming or sort, recording a session (a point and shoot thing), I don't think any rocket science might be needed of which we can find volunteers among us to do that.