Rather misleading title, since it's not actually about a prize, it's about a prize that some guy thinks should exist.
The problem of allowing one-way rentals is not one that I think Zipcar is particularly interested in solving. No matter how you slice it, it winds up leading to a less efficient allocation of parking spots than the current one spot per car model, it increases the probability of a pissed-off customer who finds that the car from his local spot has semi-permanently vanished to four hundred miles away and nobody is currently planning on one-waying another one in, and it wouldn't necessarily lead to enough increased revenue to make these two downsides worthwhile. In fact it might well lead to less revenue, since someone making (say) an overnight round trip could just book two one-ways instead of a return and wind up saving money.
I don't think the title is misleading, and it definitely isn't intentionally. Half of the post is an email to Zipcar proposing it, and the title needs to be short.
I think the other issues you identified are real, are solvable, and are addressed in the post.
The post explicitly says not to allow a one-way trip to empty out a lot/neighborhood (permanently vanished car). An "I miss this car" button on the Web site would make it easier for Zipcar to know when to reward someone for parking a specific car type in a given lot.
In many cities, Zipcar has dozens of cars in the urban core. The use here is going from Seattle to Bellevue or Mountain View to San Jose, not BFE 1 to BFE 2.
I agree that Zipcar hasn't been not particularly interested in solving this problem. I think that may change now that they have competition in most markets.
As far as less revenue, most of the trips this enables aren't happening today because so few people spend $60-$80 (day rate) to drive 30 minutes then park for 8 hours.
If someone could book two one-ways cheaper than a round-trip, it would be because it's helping Zipcar. If my driving balances out two one-way trips booked earlier, I'm helping reduce Zipcar's costs by slightly altering my behavior (driving different cars).
What if such a system would only allow one-ways if A) nobody had reserved for any extra gap created (or the gap-cost were charged and split), and B) another person was intending to do a one-way in the other direction, with a similar-grade vehicle? ie, only fairly-precise trades / N-way rotations. This, of course, makes one-ways to/from small cities very hard to achieve, but it gives you one-ways in cases where it does work for almost free.
The problem of allowing one-way rentals is not one that I think Zipcar is particularly interested in solving. No matter how you slice it, it winds up leading to a less efficient allocation of parking spots than the current one spot per car model, it increases the probability of a pissed-off customer who finds that the car from his local spot has semi-permanently vanished to four hundred miles away and nobody is currently planning on one-waying another one in, and it wouldn't necessarily lead to enough increased revenue to make these two downsides worthwhile. In fact it might well lead to less revenue, since someone making (say) an overnight round trip could just book two one-ways instead of a return and wind up saving money.