Parse /proc/cmdline and get iso-scan/filename param value;
Somehow determine boot device (I don't know how) and mount it;
Bind .iso file with losetup.
Later on (before disk partitioning i.b.) installation process asks for unmounting mounted devices. The question must be disabled for a device containing .iso image.
Need TODO:
/proc/cmdline
and getiso-scan/filename
param value;