All good points from where I sit. I'm not sure if it's because the metal community is more friendly to each other than in some genres, but I've had really good luck dealing with individual artists.
As for payola...well, I used to track songs for various bands and labels (most probably know, but for those that don't tracking is the process of contacting PDs and getting them pick up a tune for rotation. In exchange, you provide payola in the form of tickets to shows, backstage passes etc. It's not necessarily pay-for-play, but pretty close
) and I can attest that it still exists despite lawsuits to the contrary.
I can't speak for what it's like outside of my chosen genre, but starting with locals is a good way to go. It's simple enough to tell a local band that you'll play their tunes on your show if they guest list you for a show or two. Even bigger well known bands I have contacted over MySpace have told me to play their music all I want. I've helped local bands sell CDs to people on the other side of the world that would never have heard of them any other way.
Of course...the fact that I don't make any money, don't play ads on my show, and generally keep everything completely free makes them see that I do it for the love of the genre, not to make money or get comp tickets and the like. I've yet to contact a band (of any fame level) that is against me playing their tunes without paying royalties because my listener base is small and no money is changeing hands or going into my pocket.
Obviously for bigger broadcasters that have a revenue stream this isn't as simple. At that point the label gets involved and wants to see that royalty money. It is after all their property. Every band I've talked to about playing their tunes on my show has asked about my listener base and whether I collect any revenue for the show before saying "yeah dude...play that shit out!"