Well... I pinged the dealer straight back (had thought I might have another week or two to think about it given the original prov.build date of week 9) but he came back with 'as it's only 2 weeks to the likely build date it's pretty unlikely I can make spec changes at this stage'. I suspect that the reason build dates get jiggled around is that there is a massive ERP process going on where the myriad components for the extant orders are ordered in bulk and then as the supply chains all sync up the precise spec for each car determines a provisional build slot which is then confirmed when all the relevant bits are available.
I have a mental image of an order acquiring a shopping list of parts based on spec, which gradually get ticked off as bits are allocated before finally going 'I have everything, feed me into the car building machine!' when everything is confirmed available. Obviously the shorter SEAT can keep the supply pipelines the better from a work-in-progress point of view, but there's an irreducible minimum lag between 'got all the bits, freeze spec' and 'start putting car together' which seems to be about 2 weeks. And of course having allocated parts and slots, if they allowed the spec to change past that point, it would cause a bubble in the pipeline because the revised spec would have to go back through the allocation process.
I suspect (but am short of information) that the different engine/chassis/gearbox combinations may actually be built on different lines at the factory for logistical reasons, too, which is why changing from a manual 150 to a DSG 190 (like what I did!) requires cancelling and resubmitting the entire order. The setup time to build a different basic structure on any given line is probably nontrivial, which would also explain part of why the petrol DSG has only just become available to order in the UK even though the parts are all common across other models - VW group had to rejig the lines to make room for it.