1. add more twist to my plying job: run my finished yarn through the wheel again to give it more ply twist (and see what this does to the diameter and "feel" of the yarn).
Here are some "before and after" photos. By running the yarn - quickly - through the wheel again, I managed to go from 16 twists per inch to about 20 TPI, which is I think the limit of the amount of twist to have. The yarn shrank in diameter a bit, although it's hard to tell because it's not a very even spinning job and I didn't measure the same piece of yarn "before and after". But my impression is that I moved from about 10-12 WPI to about 12-14 WPI. The replied skein was squirrely and twisty, but the new twist set in the soak and it now hangs straight once again. The yarn is a little harder, but still quite squooshy. Southdown can handle a lot of twist - much more than Cheviot or Suffolk, I'm seeing now. I've recently tried plying Cheviot at 20 TPI and it's a lot harder. Suffolk cable-plied at 14 TPI is basically string.
[10-12 wpi, 16 tpi before replying]
[a little tighter and 20 tpi after replying]
2. ply in some nylon: I have some wooly nylon from my serger, and I'll ply that together with about 5 g of my finished yarn at the same time as I'm "up-plying" it.
Here's a photo of the finished yarn. The wooly nylon is very elastic and thin. Plying was actually a royal pain to do, because both the diameter and elasticity of the nylon and the 4-ply yarn are wildly different. So it's very difficult to ply the nylon in evenly. It's pretty clear that adding nylon in this way is a poor second choice to blending nylon fibers in at the roving level! I don't think I will be repeating this experiment. It's quite likely more effective - as well as easier - to just knit in some reinforcement thread.
[wooly serger nylon plied into 4-ply yarn]
3. add nylon to the top: I managed to convince a group of fellow spinners to split a batch of custom-blended Southdown with nylon added in! By the mill's advice, it'll have 20% nylon, which is about what commerically-spun sock yarn has, so I'm betting that this blend will be very hard-wearing no matter how it's spun or plied. For wool purists, though, this sure feels like a cop-out...but I'm happy to be able to get my hands on some fiber that I'm pretty sure will make hard-wearing socks!