Customized loading is only available for the org accounts, pro accounts must keep the PlayCanvas screen during preloading.
We have recently added support for HTML and CSS assets which you can use combined with a templating library [like Hogan or Mustache] to build a complete UI.
We also have some code on our github which lets you use 2D sprites and fonts.
But you’re right we’d love to have more support for building UI’s in the Editor. We’ll get there, but there is a long list of things to do 