It would be nice if I could take out some of the RAID1 replicas of my root and boot filesystems for a new installation, so that I wouldn't have to raidsetfaulty and raidhotremove them before starting a new install on an existing system. Taking replicas or spare disks out of a RAID1, or one of the active members of a RAID5 array, could be supported by the installer, but I'm not sure it's such a good idea. It would also be nice to be able to create RAID1 devices with dummy (inactive) members, so that say one could install using a pair of replicas taken out of the original /, and then further replicate this device onto the devices that remained holding the original /, after the old version is determined to be no longer worth keeping. Another possility to achieve the same effect would be to allow formatting of a RAID device without formatting the contained filesystem. This would only work for RAID1, of course. For RAID5, some resizing and raidreconf would be necessary, so it's probably not worth the trouble.
I really have no idea what you are asking for, could you give a concrete example?
Say I don't have any space on my 4 disks that is not allocated to any filesystem, and I want to install a beta without overwriting anything. The way I generally do it is to mark as faulty some of the components of the raid1 device I used for `/', then remove them from the raid1 device, and then use them to install the new version. Currently, I have to do this by hand. It would be nice if the installer would let me do it, though. Taking disks out of a raid1 device is no big deal, as long as at least one replica remains (if not, the raid device itself may well be deleted).
This is not an operation I want to support from anaconda. Perhaps the distribution config tool group can write a userland tool for RAID/LVM management.