I stumbled on a RJ50 this friday so now I have just made myself an Monitoring cable for my old APC Back-UPS 550. It's a regular USB cable that has a RJ50 in one end. It looks like a rj45 ethernet, but with 10 pins instead of just 8. However, only 5 of these 10 pins are used, so its obvious that APC only made it like that to earn some bucks selling their weird cable for $35.
Pictures of another DIY here and pinouts here.
I hooked it up to a machine running Proxmox with Webmin and apt-get apcupsd apcupsd-cgi. I found the Webmin package here tweaked it like so.
Works like a charm.