I think they are soldered in, which would make that tricky. If this is offered at all then I think it should be at order time (and with a very limited set of combinations of choices).
Depends how much more but I would be open to considering such an opportunity.
As you say, there is limited scope for more RAM - so significantly more RAM can only really come with a future hardware revision. So we would mainly be talking about a larger eMMC drive.
This has already happened. [MyL5] Australia/New Zealand - #83 by irvinewade and the post after it. uSD is disappointingly slow. eMMC is “OK”.