Initially, I was able to update frimware using OEM nec flasher through usb2.0 connection and didn't have any problem flashing 2.16 to 2.17 to 2.18 NEC firmware when it became available. I know it wasn't a recommended method for flashing, but since nec3500ag drive was detected by the flasher via usb2.0 I just treid flashing it and it worked.
So far, I've used binflasher 1.3 and 1.3a to update nec3500 firmware to modded firmwares and it worked so far. I do own a mac and a pc, this is the main reason for having nec3500 in a external enclosure and to be able to access DVD-RW drive on both platforms. I however, haven't tried flashing on a mac, yet.
edited: Just tried binflasher 1.10 and it works via USB2.0. BTW, my fireware version for prolific 3507 chipset is one dated 090704. Not sure if that makes a difference.