I've requested it three times with mixed results:
1. Asked the captain directly (he said he normally doesn't have it on but he would turn it on for this flight)
2. Asked FA in E-, FA called the flight deck, flight deck said Ch 9 wasn't working (seems like a long shot that Ch9 wouldnt work but the rest of the audio entertainment was working).
3. Asked FA in E+, FA called the flight deck, flight deck just said no.

^ Good work flight attendants for calling the flight deck for pax.
Sounds like asking while boarding is probably the best way, however.