Web UI has a “mechanism” to load and expose some Polymer stuff for compatibility purposes, as can be seen here.
We've already did a contribution to ensure that mixinBehaviors could be used. Please see
There are still some mixing Polymer capabilities missing from this compatibility effort, one of them being the afterNextRender. The aim of this ticket is to try to identify and contribute to WebUI the missing needs, taking Aspera Connector as the driver.