Yes with a caveat. The now playing screen works very well with multiple controllers. The library works, but each time you switch clients you need to click the refresh button at the top. See this link for an explanation why: Roon Extension: Roon Web Controller v1.1.1
I like this idea - depending on the licensing. All of my code is MIT license, so I use the Material Design icons which are also MIT license.
The App Store idea has been hinted at. There is another extension called Extension Manager that tries to provide that capability. EDIT: see @Jan_Koudijs post!