In the absence of a reply I decided to mock it up. I used an HS210, a regular three way switch connected to a desk lamp with its own switch. The HS210 worked flawlessly both receiving the power and switching it through one of the two shared wires or supplying power to the lamp. It even worked when the lamp was off, i.e. switched the power appropriately. In all cases the state of the power to the lamp was correctly reported to Kasa.

Yes a single HS210 works flawlessly with a regular three way switch.

To Do: Test with a three way dimmer...

I did the same today and no issues. TP Link actually includes this in their FAQ for any wiring setup where only one switch has a neutral bundle so they must be aware that it works.

I set this up today! I used one HS210 on a 3-way and it works! I'm pumped. Now I have another switch I can use!