Unfortunately, I don't think there's an easy way to do what you want. Wake-on-LAN (or WOMP) requires a specially formatted packet; by design, it's hard to wake a computer "by accident."
Even if you could convince a web browser to include the magic string (which includes your server's MAC address) in the request, it wouldn't work, because the server would need to be woken up by the first packet in the connection (TCP SYN), which doesn't contain data. (And even if you did manage to coax a web browser into including the magic string early enough in the request that it would fit in the first packet, and convinced its OS to include that data in the SYN, it still wouldn't work for at least the first request, because the server's OS would never see that first packet.)
Probably the easiest hard way would be to set up some device between your server and the internet (like a small wireless router) with a HTTP proxy to your real server, and somehow set things up such that whenever a request comes in to the proxy and the real machine isn't awake, it sends the magic packet.