Currently Sabayon doesn't support installing on these systems, since the version of anaconda it's based on doesn't comprehend dmraid, and the only way to install at the moment is to install onto a non-RAID device.
I've tried hacking the anaconda scripts on the live system to get it to install, but keep hitting blocks. Currently I can get the devices seen by slinstall, but it still bombs out. Even so, this is pure hacking and it's only suitable to me atm - not to a general community. At this point I've taken the hacking as far as it can go, and can't get any further
The CVS version of anaconda does support dmraid. Would be great if the Sabayon team could pull the dmraid stuff from the anaconda CVS into slinstall so that users can easily install into these types of systems.