I found another post (2009, on fedora-virt list), which is relevant:
http://www.redhat.com/archives/fedora-virt/2009-December/msg00032.html
I have a CentOS 5.4 guest x86_64. If I remember well
(...)
hot add of scsi disk should work if there is already a scsi disk into the guest (and if the guest supports it).
Well, in my system there is no scsi disk on the guest (the guest only sees the img on which it runs), and I don't know how I could get one there. :-/
- Jussi