Usually the thing with button combinations should be handled by uboot. ADB probably tells uboot to go into one of the three default modes - that is recovery, bootloader or normal system. But since these modes don't seem to exist in uboot, it defaults back to the normal system. Somehow, in order to apply updates, the system can still reboot into those modes - you could of course decompile the update check apks and have a look around there, but you shouldn't do that (unless that mode comes OTA as well). As of now, adb reboot bootloader and adb reboot recovery don't do anything apart from a plain reboot.
The easiest solution seems really to be to get on everybody's nerves each time they make an update, ask for the new version of uboot that they modified, get the different modes back in there and dd it just like they do.
/recovery and stuff is all there - it just doesn't do much AND there is no normal documented way to get in.