They don't exist yet.
Aside from the Inhalater (which, even after many months, seems to be having trouble becoming a proven option) the Cera with loose leaf cart is the closest thing. But, though it has the pen shape, it's definitely bigger than a pen, and heavier.
It can be a powerhouse, but it's more of a "vapor enthusiast's vape" in my opinion, having a larger than average learning curve and caveats like weight, being mindful of heat buildup, and cost. I'd hesitate to recommend it to anyone new to vaporizers even though it's my favorite.
Like you mentioned, the Solo is the way to go for the average use case.