Es gibt keine größere Illusion als die Meinung,
Sprache sei ein Mittel der Kommunikation zwischen Menschen.
(Elias Canetti)

There are quite a lot of things that have to work together properly, to make a PXE-Boot-System work. If you really want a fully-qualified operating system, you will have to export a filesystem, for example via nfs, have a special kernel that can use this filesystem, and act as a dhcp-Server and NAT. It can be complicated to configure all this.

In a large network, you may want to have a fine-grained configuration of all of this. But usually, you are in a situation where some computer chooses to strike and you cant get any bootable medium. Well, surprisingly, still, the most safe way to get something to boot at least somehow is - besides burning a CD - to boot it via PXE-Boot, as far as I see. Maybe because this is some standard that most computers support (I am afraid that this will change as soon as the first routers that dont support IPv4 anymore will spread). So it would be desirable to have some software-package, that allows you to manually turn on an administrative dhcp-server on one of your ethernet-devices, with a linux-live-system that can install itself, such that you only have to plug in the computer you want to boot and then get a system to work with, and maybe to install directly.

There is a package that does something similar, namely ltsp-server-standalone, which create an LTSP-Architecture - but thats still not what I want. I want a bunch of servers and their configuration which I only have to turn on when I want it, and then "just work", passing a live-system (like on the ubuntu livecd) to all computers doing netboot, and routing internet connections.

It should be comparably easy to create such a thing (maybe it would take long to make it stable). In the end, there are a lot of howtos to achieve a Netboot-Livesystem. The parts are there, they only have to be put together.