My Linksys router is quite old (3-4 years), so I'm fairly certain that it can run a custom firmware, but I've never bothered to try. I've heard of this specific firmware for my router before but have never gotten around to trying it out. Maybe this weekend I'll give it a look over.
I haven't noticed such an option in the WRT54G's original firmware, but it is possible that the Tomato firmware could do this. Having such an option would certainly lessen the chances of a hacker coming in.
Tomato does let you control wireless transmit strength (and a host of other things). Some of the firmwares are fairly tricky to setup and require a good deal of technical knowledge (OpenWRT is one that comes to mind), but Tomato is pretty much dead simple for all the stuff regular users would want with 90% of the stuff advanced users want. Installing it over the base Linksys firmware on the WRT54G is a 5 minute procedure that basically consists of selecting the file and pushing the "upgrade firmware" button.
I spent way too much time configuring my network and researching various firmware options when I first was setting it up (I tried three different firmwares before settling on Tomato), so I can probably answer any other firmware-related questions you have.
But as other people have been saying, you're taking plenty of precautions already, so I wouldn't worry about it.