You might be able to rig something with
kexec is intended to boot kernels, but nothing's stopping you from
kexec'ing to any binary (a "statically linked" binary that doesn't depend on libraries or an existing running OS and is meant to run under preboot conditions) you want, whether a bootloader or even memtest86 - that page talks about some of the traps you may fall into. E.g., if it's a text mode bootloader, you should switch back to text mode before issuing
If you're using Debian the
kexec-tools or whatever pretty much performs all the normal shutdown steps, including switching back to text mode, before issuing the
kexec won't do is boot a disk or disk image, but you could extract the bootloader from the CD and pass it to kexec. If the boot CD is in the drive I don't see why it wouldn't work but I haven't tried it.
Unpacking an .iso in RAM and getting the .iso's bootloader to run from it without the bootloader being aware of the situation would be technically possible, but extremely difficult. I don't know of any utilities capable of that and don't believe any exist.