Thanks for the reply, but...
This article briefly describes the existence of these pools and also that there are manufacturers who offer multimodem solutions. This is not what I'm looking for: I want to build a modem pool using standard (USB) modems, not to buy a propriatry solution.
This document describes howto dial up into an existing modem pool, which is also not what I'm looking for.
I've looked into pppd a bit more, and it seems that it supports plugins. To enable the plugins the ppp deamon offers some hooks to its internal functions. A plugin can hook on to these to overwrite the internal function with its own. Hooks exist for PAP/CHAP authentication and some other functions, but not for the function which initiates the dial-out action. Would it be an option to patch pppd to add some more hooks to its code?? Perhaps someone has allready done that without me being able to find it...