This is far beyond the USB spec, but I was wondering if using one or more USB hubs/repeaters would do the trick.
Yes. There's a device commonly called a "USB active extension" that is nothing more than a one port bus powered USB hub on the end of a 5m (the maximum allowed by spec) cable. We have used two or three of these to get from a ceiling mounted projector back to a computer sitting in a corner.
Another long-haul USB option is a pair of devices that convert the signals from USB to something else and exchanges them over cat 5 cable. A bit more pricey, but a bit longer haul, too (100m or better, IIRC).
I was wondering if anyone had experience with serial line drivers?
Lots of folks, as this is the way the majority of computer communications used to be done. There are specs on the cable for rs232 (EIA232 might be a more correct designation for the spec), but for 60' you can get 9600 baud on just about any old wire, using appropriate line drivers/receivers like the MAX232 family.
could I put one in between the tx/rx lines between an arduino mini and the mini-USB adapter?
You'll need one at each end.
I've built a few Arduino-based boards of my own design, and I use the MAX232 when I need true rs232 connectivity. Works great.