I think you're in a bit of a bind here. I think you'll have an easy time getting "does this work for you?" feedback, which is crucial. However, I think you'll find it very difficult to find constructive, "here is how you can make your letter awesome" feedback. Let me explain.
You're trying to be attention-grabbing and evocative. Notice that this is basically a self-marketing strategy that you yourself are selecting; you could choose a different one (as rianjs wrote), but this strategy could be good for you. Attention-grabbing is what you want, right? Well, yes - but it's also a strategy that could misfire pretty horribly. Because if the recruiter doesn't fall for your letter hook, line, and sinker, he's very likely to reject it outright for being unprofessional, gimmicky, and/or inappropriate. It's basically an all-or-nothing gamble, and potentially a very risky one. Here's an article about absurdly inappropriate manuscript submissions; it's fun reading on its own, but I'm linking it here to illustrate that a lot of peoples' idea of "unusual" and "creative" can be a major turn-off for the person at the other end. Not only that, the weirdos are so weird and unprofessional, they often make the recruiter allergic to any sign of mold-breaking "creativity." "Just FOLLOW THE INSTRUCTIONS," they are muttering to themselves, "don't be like the guy who sent his submission in inside a pizza box. Don't be anything like him." That can be a lot to get past.
So, you're looking for two things. You're looking for a reliable indicator of whether or not something you wrote is absolute dynamite. And if it isn't, then you want someone who will turn it into dynamite.
The first is easy. Ask a bunch of friends. People whose opinions you trust and respect; people with different tastes and sense of humor; people who won't be afraid to tell you if your letter isn't as awesome as you want it to be. You don't need professional writers - you need friendly dynamite-detectors, and your friends are fine for that. If they don't really like it, you can rest assured that the recruiter is likely to be far, far harsher than your friends.
The second is near-impossible. There's nobody who really does what you're looking for as an occupation or a hobby, except maybe advertising executives... And you're trying to create something really unique, that has to be as near to 100% effective as possible. Even if someone does give you constructive advice, you need to make sure that advice really really works. And frankly... if there were somebody out there who could spin up unique application cover letters from not-so-great into pure dynamite... well, I don't know about you, but I know I couldn't afford him. :P
I might be misunderstanding just what it is you're trying to do with your letter; maybe it falls into some category of writing that has an established community of practitioners. If my above comments didn't hit home for you, then consider the following question: what categories of fiction, non-fiction, or marketing might fall closest to what you're trying to do with your letter?