I'm coming to this discussion a bit late, and I'm not the real economic Guru in the Design office, so please forgive me if I say something mind bogglingly stupid. I'll point Isildur at this thread when he gets in to the office and he'll correct any errors I make.
As a couple of people have pointed out, the restriction of 10 lots per account per server exists to push people away from self sufficient production. The economy is built on the exchange of goods and services between characters. If you can manage the whole production chain yourself, all you're doing is putting good into the economy. What tends to happen, when that's possible, is that the only economy players are the ones selling end products: ships and outfitting. We want the economic middle man to be a viable option.
If one player can run an entire production chain, that chain will be run near or at 100% efficiency. We don't want that to happen. Inefficiencies in the economy, sometimes referred to as economic friction, can be exploited by clever economic players for their benefit. This happens in the real world too, and while the value of exploitable economic friction is questionable in the real world, it makes for interesting game play (and no one will starve because cannon ammunition costs 5 doubloons more per unit in our game).
Because large pieces of this economic friction depend on resources being located at a distance from each other, we cannot allow the mailing of items from port to port in game. Mailing money is something we might consider (because you take your money with you when you book passage to another port), but it makes me a little nervous.
We could, in theory, construct an escrow system that allowed you to place goods in escrow for a particular party in a particular port. Obviously we would need restrictions on how many goods you could put in escrow, where they could be transfered, etcetera. This is something we've talked about, and it's possible this is a feature you could see one day. It would require some non-trivial work to design and make sure we plug all the potential loop holes, and then it would require the actual code time to implement. It's not very high on our current 'giant list of things we want to do' (which, incidentally, has multiple years of work on it), but if you feel this is a particularly important issue we can reassess that.
Finally allowing all characters on the same account to access the economic structures of that account is, honestly, not something that had occurred to me. Although I bet Isildur has thought about it already. Off the top of my head I can't see any huge design problems with doing that (obviously we'd have to look carefully at it and close potential loop holes). On the technical side this would be a pretty significant amount of work. Without going into detail, we didn't set structures up in way that makes this change easy. We could do it, as one of our developers likes to say "we're working with computers here, we can do anything" but it would be a major change. That said, I think we can put it on the list. How important is this ability to you?