So Generals only supports two types of prerequisites: Objects and Sciences. However I think that it could be very usefull to some people if they could use upgrades as prerequisites, so this is what I propose:

You can grant sciences through crates, and as shown by the saboteur, you can have buildings pick up crates. Using an upgrade, you can an OCL from the building, so you buy the upgrade, spawn the crate, have the building pick up the crate, and then grant the science, which is then used as the prerequisite for whatever.