Several things to consider.
You cannot create a wireless network for guests if the AE is bridged. It has to be the main router in the network.
You cannot use the existing TM backups on the USB drive.. if you plug into a network it will need to be wiped.. and only AC model can produce reliable TM backups.. the earlier models are not reliable for TM backups.
These two factors alone may be enough to make you reconsider.
If you can create a guest network.. and you can overcome the limitations of airdisks.. then any person on the guest network should not be able to access the disk drive.. but if you wish to go ahead Bob Timmons might be able to confirm it.. I stay away from USB drives on routers.. I do not and will not use a router as a NAS.. it is underpowered and simply unsuitable long term.. a cheap single disk NAS is many times faster.. more reliable.. and has a marvellous thing called backup.. no USB disk or even internal disk in a TC can back itself up. .nor can TM backup network drives.. so any data you store on a USB drive is going to be gone one day.. unless you go to great lengths to back it up .. to where.. another USB drive plugged into another router somewhere.. the definition of slow.. should be copying files between two usb drives in an apple network using wireless. You will experience new levels of pain and suffering.. likened to the belly of the Sarlacc.