Hi robotecho,
i don't know much about your oberheim and yamaha rack synths, but if they are able to receive program change messages, or being able to receive control change messages, it should be fine.
The goal is to know what type of messages your rack synths are waiting to receive (sorry i can't help you further for these as i'm lacking knowledge on this specific hardware)
But once you figured out, on the keylab mkii,when on user mode, it is possible to configure switches under faders, directly on the keylab mkii, or using the MIDI Control Center Software , available here :
https://www.arturia.com/support/updates&manualsswitches can be configured to send CC number(switched control) with a min/max value on the channel you want, this function is fine to browse through presets.
It can send also program change messages (with a program number, bank lsb, bank msb) in order to load a specific preset just by pressing one button.
Hope it helps a bit.
cheers,