Hi Chris, Sorry to here of your squealing brake problem. Have you tried contacting SJSC directly for their advice on what works best. They must have a load of experience. My own solution to this, albeit not with Rhyno rims,and apart from cleaning the rims and pads occasionally, is to find myself a long, shallowish hill and find a speed/ brake pressure combination where squealing doesn't occur. I maintain this for a while and afterwards the brakes don't squeal. It seems to bed them in properly. Anyway, that works for me.
With regard to rims, You can buy 26" Alesa Endeavours from
http://www.spacycles.com . They're cheap too at £14 each approx and weigh a little over 600gms and have a wear indicator.
I haven't had first hand experience of Mavic rims cracking. My MTB tourer has a Mavic D521 rear rim with no trouble so far, but according to people I consider to be experts, such as CTC's Chris Juden, they do tend to crack. It may be that the new 700 series alloys are not prone to the problem and I have noticed that Thorn are specifying 721 rims on the new Raven, so maybe they've cured it.
Hope that's of some help, regards, Lucian Proctor.