I think you'd need to somewhat disguise what was going on

Agreed.

I'm not sure you appreciate the dynamics of messaging on these sites.

No, trust me, I know this part. :p But I see what you mean; my afterthought about how it would reduce their message quantity missed the point.

particularly if the messages are higher quality (which they will probably tend to be if they are expensive to send)

I wasn't assuming that would be the case, although

recipients receive some or all of the cost of messaging them

this would mean that hot people would mostly receive messages from other hot people. Which, according to your link, would be preferable for them. Interesting.

Okay, I retract my immediate rejection, at least enough to admit that I'm curious about how this would pan out.