Does the Si4731 / Si47xx provide the necessary info to fetch left/right demodulated audio levels?
I do not believe it does, but you can just process the digital audio stream to derive whatever style of metering you desire.

Audio metering is a can of worms, there are literally dozens of different standards worldwide, VU, PPM (But which scale?), DPM (But what dynamics), Loudness (There are at least three), nobody is going to bother with that in a simple radio chip, far better to leave that to the customers processor if they really want it. 

Bring the I2S or whatever format makes sense into your micro and do the sums on it.