Hey,
This is not currently possible, we de-scoped 100% AudioSource API parity with AudioClip to get the AudioRandomContainer out ASAP. One of the issues with the length is what should we report for an infinite playing container, for example.
Unfortunately what @CodeSmile suggests and what you implemented above is the best way to achieve what you want at the moment.
We are actively working on lower level changes that will allow us to address things like this in the future - mainly looking at separating/moving AudioSource API’s that should really be on an AudioResource instance. Sorry for being vague, just know we are thinking about your exact problems!