I am quite a bit relieved to report that I have managed to coerce Debian Live into working quite similarly to a Knoppix Poor Man's Install.
Which means that I can use it with some degree of efficiency.
I byte-copied the ISO image onto a USB stick, created an ext2 partition on the rest of that stick, and created a live-rw file image there with an ext2 file system on it. Did some installs on it, it worked quite fine under qemu-kvm (necessarily 64 bit version).

So far, so good, but what about Poor Man's installs? I copied the /live directory from the ISO file onto a FAT32 partition (/dev/sda1), and the live-rw file onto an ext3 partition (/dev/sda8 ). Then I created a /boot/deb760_live directory on my boot partition (here /dev/sda6), copied vmlinuz and initrd.img from the live-directory there, and modified old legacy grub's menu.lst with an entry for Debian Live:
Code:
title Debian 7.6 64 bits live sda1
root(hd0,5) 
kernel  (hd0,5)/boot/deb760_live/vmlinuz boot=live config quiet splash  initrd=(hd0,5)/boot/deb760_live/initrd.img persistence  keyboard-layouts=no
initrd (hd0,5)/boot/deb760_live/initrd.img
And it worked! Debian complained it didn't find the persistence file at /dev/sda2 as it expected, but got around to mounting it from /dev/sda8 anyway. It passed by a file with the same name at /dev/sda1 - that's fine with me. Documentation says that several such files may be mounted, provided they are in separate directories and have different, compatible, mount points defined.

One of the great attractions with Knoppix is that you can always do a remastering from the version you use. Debian Live takes an almost diametrically opposite approach, making the creation of new images extremely simple, but with little interest in using them and remastering them - it seems to be more designed as a vehicle for Debian installs. We'll see how this works out.

The iso-hybrid image doesn't work on USB in many BIOSes, and unetbootin which is recommended as an alternative for creating bootable sticks, crashed for me. So I'll rather work around that, using Knoppix methods, I think.