Before you read any of this, check out Automatic Stocking Adaptation for Heel. It probably won't work if you're using a UBE body because UBE bodies often require two sliders to be changed and this mod currently only supports automatically adjusting one slider, but maybe you're using a different body, or the mod author added support for multiple sliders by the time you read this.

The solution I used was equipping a mostly identical item in a different slot, giving the high heeled (HH) model a higher priority so it displays instead of the non-high heeled (nHH) model when they're both equipped at the same time, and treating the HH stockings and the High heels like basically the same item when equipping them in game. In effect this means wearing the nHH model like underwear that gets covered up by the HH model whenever you want to wear high heels.

You'll need to use Outfit Studio, Bodyslide, nifscope, and TES5edit. The part in outfit studio is lightly complicated but you'll probably be fine with the rest of it. Probably... Oh you'll also need the item to have an alternative variant of an armor piece that you're willing to overwrite, I didn't bother learning how to add another item in the game. As I write this it occurs to me I probably do, but that's really not the important part of this guide anyway.

You'll need to understand (ish) how to copy the bone weights from a reference object to the armor. This tutorial will help:

https://www.youtube.com/watch?v=tvJGbKpVY8E&t=1008s

Step one, we have to make either a HH model or an nHH model, whichever your armor setup lacks. Go into outfit studio and open up the .osp file associated with the stockings you want to fix and immideantly save it as a new file. The display name isn't super important, but you'll need to change the output name to the output name of whatever armor piece you're going to sacrifice to overwrite with the new stocking model. Hit the "to project" button and then the "Slider Set File" field's "Browse" button to specify where your new osp is going to be saved to. 

If you watched the tutorial, you probably know how to use sliders to change the shape of a piece of armor to match high heels and vice versa, so do that, copy the bone weights, etc. The only thing not covered in the tutorial is how to deal with bone weights in the leg/thigh section. To do that, go to View->Enable Textures, then use the mask tool (it's in the toolbar at the top of gui) to mask the foot area of the armor, then import the body model as a reference and copy the bone weights from that.

Then follow these guides to change the nHH armor piece to a different body slot than the HH armor piece,

https://www.loverslab.com/blogs/entry/14631-the-basics-of-body-slots-for-skyrim/

https://www.loverslab.com/blogs/entry/10352-changing-body-slots/

but add the original body slot to it's ARMA value and give it a lower priority than the HH piece. 

Go into body slide and build it. It should now work.