Just my opinion here, but it is when these "store" modules start trying to control inventor, that they become tedious to use. This has stopped me from purchaseing several of the better shopping carts.  A simple way to signify something is on backorder should surfice (check box in product table that indicates backorder with a display and not display option).  JMHO.  I work for a large company , our product line consists of over 1oo products and we have warehouses in US and the UK.  Our web department gets an email notification from 4th shift (our manufacturing/production management program) and we then take appropriate actions.  Givent the fact that an iventory control program, to be effective, has to integrate to purchasing, warehouse control , have stock safty level capabilites,