My guess would be that a meal order is not a change that would touch seating, because basically the two are a separate functionality. So from a programming point of view this would add unnecessary complexity (which causes the developer unnecessary work - hence they'd avoid it).

Only an educated guess though...

Why don't you just book your outbound meal at 7 days and then keep a close eye on your booking for the following few days (e.g. through the myflights app)? Worst case you can then still decide to pay for your return seats.