The advantage of 3 pin fixed voltage is that supplies can be made ridiculously simple and efficient. So yeah, a smart supply for each outlet would be simpler for the users and auto-negotiation could easily be built in. The problem is that now you have a single power supply design that has to be able to do 5.2V/0.1A for your USB-charging ebook (Sony 505, PSP) as well as 100V/5A for your PC. That's a pretty hard call for any power supply and it's going to cost you - not so much dollars (I am happy to spend your dollars) - it's going to cost efficiency. Either your "single supply" will actually be two or more supplies in parallel (lots of size and extra devices), or it will be ludicrously inefficient for most things. I think the best case would be that it would itself run on 5V and be able to power a USB device or two off that, only bringing the main supply up for tasks that need more juice.

But when you can design for a narrower range it gets a lot easier, and you can be more efficient. For example I use little 12V/1A switch mode supplies for various things. The 21V/1A only ones are ~85% efficient, but the same company also does a 5V-12V/500mA supply in the same case. It's 70-80% efficient. So I run a 90% efficient 12V/100W supply (that has a 1W parasitic load, so it's not so good at low power levels) to drive all the random 12V junk that hangs round my computer. I really should get more 12V lighting to use some of the extra power.

As far as HVDC power transmission, the early examples used rotating converters - literally motor-generator pairs on a common shaft. That should give you an idea of the date :) I grew up in NZ where there's been an HVDC link about 500km long since the 1960s. http://www.abb.com/cawp/gad02181/c1256d71001e0037c125683400270fa6.aspx The guys who built that also did work in Brazil (the ideal country for it, there's big mountains on one side and lots of people on the other).