I see the reason why we would need such things, and I am a massive fan of web components and frameworks like Montage. But I also see the other side, where realistically we are just trying to reinvent a more low level language on the web browser.

In that case an x-calendar element is conceivable. In fact, the date picker could use that as its content.

Luckily, Brick is open source, so you can file an Issue about it, and if you're interested, even submit a pull request yourself.