Racktop bags work well on recumbents with rear luggage racks, just like they work well on DFs with rear luggage racks. I use a Carradice Prima on my Streetmachine for day rides, and an Arkel Tailrider on my Optima Baron.
I have an old Topeak tri-bag mounted 'backwards' on my Streetmachine, on the boom in front of the stem (under-seat steering). That works well for quick access to nibbles and an inhaler.
Recumbent-specific luggage has its uses, but if the bike will happily take the normal stuff, then it's a lot of extra expense for the same capacity (and often disappointing waterproofness). Especially if you have upwrongs that you could potentially use the luggage with, too.