Question re future test setup

[UH]

Just a question re an eventual test setup for testing 13.04ppc betas.

Is it possible to do this: I've a running (nicely) Debian sid ppc
setup (which is used by my wife). Now when i mount an iso (by
Gmount-iso for example) could i install on an external hd the new
system. Schematically: Debian runs -> mounts 13.04 ppc iso -> from
where the installation starts to /dev/sdc ?

Or is it mandatory to boot from the installer? TIA, Uli