Urbaneve. I am not familar with the Zena so I took a look on line. I don't think you will be happy with a slung pony with that BCD for a couple of reasons. First is the weight of the tank/regs themselves. My understanding is that the Zena is not a robust BCD and I would be concerned of the impact the weight might have on the seams and fabric. Second, I see no place for a true hip d ring. Hooking into the front of the BCD may ride ok underwater but it may prove a bit irritating on land. If there are any tabs, my Islander had these, you can use a quick link to hook in instead of a d ring. A final option is to sew on a tab for a quick link. I did this with my Islander to add a bungee loop to sling a pony pseudo sidemount style. Last if you do sling, consider a 13 cf and at most a 19 cf tank at most.
But all this considered, you might prefer a tank mounted pony.