Considering a good portion of the players don't have a line out, and the majority of the players with a line out have it inaccessible unless you buy dock hardware, I'd suggest it's much more likely external power is probably not correlated at all to use of speakers.
If charging correlated as strongly to speaker use as you seem to think, we'd have far less people asking about how to charge without mounting, because they could just use the PC to play the music if they wanted speakers rather than to continue listening through their headphones with a minimum of effort.
I'm not saying it doesn't correlate at all, I'm just saying it's a bad assumption to make. What you've offered is not a solution to this problem. It's a solution to another problem, that if it were implemented, would provide a *workaround* to this problem. There are already other things the user could do to work around this problem, so please limit your suggestions to things that actually specifically address this problem, rather than just trying to pitch other ideas that would "somewhat work, but really has a different goal altogether."
We really don't need many threads here being regularly posted in by people saying "What about my idea, which is *somewhat* like this one, but only in special cases!"