You can load drivers after the machine is set up. Though Astaro cannot be responsible for the proper functioning and/or interoperation of those drivers. Your problem is a chicken and the egg one: how do I load the Perc drivers when the machine hasn't been installed yet? You point out a debated feature request: the install should have a point where it prompts you for drivers from a floppy/CD. But they would argue that you shouldn't be loading drivers that have not been tested by them. But I think that RAID drivers are quite important, especially on hardware as popular as Dell. So hopefully they will see this post (who's kidding who? they WILL see this post...) and they will add it to their driver hit list for the next release. Keep in mind: they don't just add the driver to the CD; they have to test it, as all good firewall manufacturers should.
If you are Linux fluent, you can try adding to the oss_inst.dat file (a tar) on the CD to include the driver; but again, they cannot be responsible for that (you will have to be). I won't go into an exposition on that, since you may not be doing it...