Hmm... the fact that you want a tech to go on-site, pop in a boot-able cd, and expect it to have nic drivers built-in is going to be very very tough.
At Hershey, we have a common desktop architecture where we have a finite # of hardware configurations. So, i was able to include the drivers needed for every hardware configuration in the CD.
To expect a CD to boot and be able to recognize and use the built-in nic of any random computer is going to be incredibly tough, if not impossible.
A thought would be to include drivers for a bunch of popular chipsets and nics, but to also have each tech carry a usb nic with them that you know the CD will be able to recognize and use.
You mentioned 'extend the lifetime of some older laptops' so that sounds like you might also want to run the cd on a work-laptop which would then have a known hardware configuration that you can plan for, with regards to drivers.