Well, I've pretty much proven to myself that me, magnets, and clothing don't mix, so I tried two different solutions:

1. At first, I tried to simply adjust the y-scale settings on the shins and thighs of the boots I was working with (in my case, LittleFox's Infamous Boots for Aiko 3), after which I exported the boots as a .obj file. I then tried to import the .obj file as a morph target. But that didn't work very well. Poser just sat there thinking about whether it was a good idea, or not, to import the .obj as a morph. So, I gave up on that idea.

2. The second idea was moderately successful. Again, I modified the y-scale settings of the shins and thighs (110% in my case, which approximates the StretchLegs morph at 1.0) and exported the boots as a .obj file (oh, in order to do this, I had to actually replace the bone structure with the one from the blank Aiko .cr2; from there, I got rid of everything above the hip; the bones from the hip and down I kept; I then saved the new figure in the character library, so that I may use that bone structure, again). I then imported the boots and took the new prop into the setup room. From there, I applied the bone structure I had saved, earlier. Still in the setup room, I adjusted the shin and thigh y-scale settings to 110%. I adjusted the foot and toe bends so that it seems to stay within the foot part of the boot. After that, I went back to the pose room, loaded up Aiko, removed the IK on the legs, and zeroed the figure. Finally, I set the StretchLegs morph at 1 and conformed the boots to Aiko.

Surprise! It worked! Kinda... I had to adjust the toe bend settings to get it to look okay, but for the most part, it worked! (I had to make Aiko's feet invisible because of pokethrough; it caused some weirdness with the boots, but going to edit-undo seemed to set things right, and the feet stayed invisible.)

So, I decided to see if I could repeat this success, and went through the same steps with the Infamous Stockings. I found that the stockings fit better if I only adjusted the y-scale settings of the thighs. However, I've got a bit of a trouble spot with the left shin, as can be seen in the image. I tried playing with the Joint Editor, but I couldn' make that bit of pokethrough go away, unfortunately. I don't suppose anyone would be able to solve that little issue?

Thanks for any and all help,
TheKeeper