Then you need to send lit-html over to the client also. This would be a great alternative if one doesn't already use Babel, but what's the selling point otherwise? Is there an easy way to combine lit-html's render with the HyperApp one?
Web Components and Shadow DOM make this possible. You can mix and match components that use lit-html, Polymer, Preact, etc., and mostly likely HyperApp.
I think within your own components it makes sense to stick with a single library setup for this reason, but atleast if you want to use a third-party component you don't have to rule out components that use different rendering machinery. You just have to weigh that additional download penalty.