From what I understand, no. Or if you can, it doesn't work very well. I've tried and it hasn't worked for me, so DVD-R's are required. However, I'm told you can change the booktype to DVD-ROM when you burn the discs and that works, too.
I'm not sure why a lot of people are having issues with +R when booktyped to DVD-ROM, majority of my backups are on +R and I had ran these on many slims and a few fat.
USB loading compatibility isn't very good. Some games do run fine, no harm in trying it out.
Network loading is good if you have a access to a computer to stream.
Open PS2 Loader works for USB, network, & Fat PS2 HDD (or slim HDD mods/chips) loading. Compatibility between the network and usb in OPL is strangely not the same.
OPL is a huge improvement from USBExtreme/Advance for USB loading, but is still slow at USB 1.1 speeds.