Hi Mark,
USB is quite complex and supports many different transfer modes. I did a quick scan of the datasheets and I think the USB hardware in those PIC's is limited to supporting slow speed interrupt transfers, and they are designed as endpoint devices only.
To connect to a USB disk you would need to make a USB host, this is quite different from an endpoint device, and it would have to support bulk transfer mode.
Even if you managed to write a cut-down host controller in software you would also need to implement one of several possible mid-level protocols such as SCSI, ATAPI, RBC, UFI etc.. the choice of which one would be dependant on the make model of flash disk you were using as not all flash disks use the same protocol, and some are not even publicly available standards.
Then there is the filesystem to consider. I suspect you want the flash disk to be readable by a PC ? If so you are going to need some code to handle this. e.g a FAT driver.
I've thought about a project like this in passing, seems to be quite a lot of work to do IMO. I don't want to discourage you tho!