I know that Snow Leopard brought us the option to magically wake our computers from far away if you had an AirPort Extreme base station, but has anyone found a way to do this with a standard or ISP provided router? What about using a bridge device to hook the MacBook up to Ethernet but from another room?
Macs that are capable of Wake on Wireless (WoW) and have it enabled will attempt to stay associated to the AP while asleep, and look for standard Wake on LAN magic packets. So any AP that will let the client stay associated and any solution that can deliver a standard Wake on LAN magic packet to the client should work.
Unfortunately, modern Wi-Fi security mechanisms like WPA and WPA2 may want to roll the unicast keys sometimes and often roll the multicast/broadcast (group) key, and the sleeping Mac has no support for rolling keys while asleep. So if your AP disassociates the client because it's not responding to attempts to roll the keys, you won't be able to get a magic packet to the client. You could work around this by downgrading your security to static WEP or no wireless security, if wireless security is not important for your network.
The AirPort Extreme provides three major services to sleeping Macs:
Note, though, that BSP service is also provided by current (2nd-gen, small, black, $99) Apple TVs, as well as Time Capsules, AirPort Expresses, and as you mentioned, AirPort Extremes. So if you have, say, and Apple TV on your network, I believe it'll take care of #2 and #3 for you, so you just have to solve #1.
Your suggestion of using a separate Ethernet-to-Wi-Fi bridge might be the easiest solution. If you keep the bridge on all the time, it'll stay associated to your Wi-Fi network, and now the MacBook only has to do normal wired Ethernet Wake on LAN, which is straightforward. You just have to pick a tool for sending magic packets.