You know, I've never really thought about this. =o
I think you'll be hard pressed to find a 56k router, especially one that broadcasts a wireless signal. As far as I know, they don't exist.
If they do, you'd need to have an external 56k modem which recieves the line-in (WAN), then that feeds the signal to your router, which forwards it to other computers via wireless or wired cables.
And why do you want wireless? If it's for desktop PCs in a home network, I'd really recommend against it. IMO, it's barely worth it for a laptop in a home network. I had a wireless network running through my house for about half a year, and just got rid of it. I'm glad I did. I had to cover the antenna of the wireless computer with tin-foil just to connect to the router that was less than 20 feet away (yes, it was a floor below, but even counting that as 2x the distance because of it as they say you should, it was well within the range the router and PCI adapter boasted).
Plus, if it were possible, I could see your dialup running even slower than dialup (yes, it's possible!). The so called versions of "accelerated dialup" aren't "accelerated" at all. It's impossible to actually increase the bandwidth and speed of dialup, all they do is compress images so they're downloaded faster.