This is painful (win98 and empeg tools).

Did anyone ever provide info about what commands the bootloader supports? Eg. what is the protocol that upgclient uses when installing a .upgrade over serial?

We already have the linux upgrader tool that parses/installs .upgrade files over ethernet. If the serial port protocol were understood, it would be easy for me to adapt that code to work over serial as well, eliminating the need for MS-Win to run the builder.