We are seeing "himem.sys cannot control address line A20" errors when PXE
booting some new machines.

It is possible to Control-C out of the script after choosing Maintenance
mode as long as we're quick. By looking at the config.sys file in that DOS
environment we see there is a switch /gatea20=at in the device=himem.sys
line. How can we get to this file to edit it and try different settings? It
must be stored on the ZenImaging server somewhere.

Alternatively, any other way to solve the a20 problem would be welcome.