At least, on a server, the owner has the control to spawn in some cactus.
A player playing the pack in single player has to make the dubious choice of deciding, how many km they need to wander from spawn without finding something to call it a bust before just cheating the item in.
My viewpoint is, if packs are going to rely on items that are only found in rare biomes, then they need to provide a built in backup. I was hoping AoE had this - I was quite willing to mutate to cactus using IC2 crops if possible (doesn't appear so), or a different crop breeding mechanism in some other mod in the pack. That would have been an interesting consequence to the world seed making cactus hard to get "the easy way".
As it is, I did recreate the world in singleplayer, and creative mode flew for 24km away from spawn without encountering a desert - to see if I could solve the problem by extending the world border - before giving up.
Some maps just make deserts really rare. And some make jungles really rare too (I'm not sure actually if I found any legit jungle either while overflying my 'seeded' map).