If your object only appears once in the staff, then a vary large span is reasonable. You can use 16384 as a reasonable large number.
However, if a user places multiple instances of your object on the staff, then this can cause problems. It will trigger every instance of your object to be placed at the front of later systems. You could avoid this issue in your span method:
local function do_span(t)
if nwc.ntnidx:find('next','user',userObjTypeName) then
This has the potential to impact NWC performance for very large staffs. If you don't want to allow multiple instances at all, then you could use an audit method to turn off the Span class for later object instances on a staff.
local function do_audit(t)
t.Class = nwc.ntnidx:find('prior','user',userObjTypeName) and 'Standard' or 'Span'