I want to create a new exterior area that I would like it to be accessible from the Tamriel worldspace through a transition/teleport on the edge of the vanilla map. It seems like it would be easiest to copy a small portion of the Tamriel worldspace into a new worldspace, create an area transition to the Tamriel worldspace, and then make modifications to my new area, but it's not immediately obvious to me how to go about doing this. Can someone point me in the right direction? Is there a tutorial out there that covers what I'm describing?

Lots o luck with that...I've asked repeatedly, but apparently nobody knows how to do it...they keep citing some completely unrelated bug (NPCs acting weirdly if further than 32 cells from the center of Tamriel) as the reason they supposedly can't tell me how to use the CK to load the Tamriel terrain at a given location, even if that location is easily within the normal Tamriel worldspace.

Yeah I asked something similar to this yesterday. It's easy enough to create a new worldspace, location and encounterzone data but creating new exterior cells doesn't seem to be implemented. You can duplicate one of the other worldspaces but when I tried that method there were so many issues to correct it just wasn't worth it. The only way to specifically do what you want may be to create a new worldspace, CK automatically adds wilderness cells, then copy the heightmap from the section of Tamriel that you want onto the new worldspace. I have no idea whether that would work as I don't like to work with heightfields but it's worth a go.
To create a new worldspace just go to world on the menus, then world spaces and click anywhere on the left pane and select new. Then configure that to your liking.

Okay, so, I had assumed when I posted this that it would be easier to duplicate a portion of the Tamriel worldspace than to build one from the ground up, but I gather building one from the ground up is the only feasible option? And once I do this, there is no technical impediment to adding a teleport transition to the edge of the Tamriel worldspace that sends the player to my worldspace, right? And this method avoids the 32 cell bug that Redwood mentions?

As far as I know you link it exactly the same as an interior cell. But like I said I have no idea how to create new exterior cells so unless you can find out (and please let me know if you do) you'll have to modify directly on the wilderness landscapes.

Supposedly, there's a way to make a "mini worldspace" that is based on any location in skyrim, that you can then modify (Like the Great Porch in Dragonsreach, where you can look out over the land and are technically outside, but the only way in and out is back through the door) - but so far I haven't been able to get it to "lock into" a specific location's terrain. I've tried defining a location marker, using that as the anchor point, and telling the new worldspace to use all the parent info...but for some reason, it makes what looks like a random bunch of rolling hills, no water (even if the area I picked is on the seashore next to a cliff)...

I just started making a new exterior cell from scratch. I read in another forum post that the Creation Kit tool for creating height maps is pants so I downloaded L3DT and am creating the height map there by following this tutorial:
http://www.youtube.com/watch?v=LUtqQUSTqh8&feature=youtu.be&hd=1

I'm going to be spending a lot more time fleshing out an exterior cell than I wanted to but it sounds like that's my only option aside from inserting my quest locations into an existing cell, which I want to avoid for compatibility and plot reasons.

The main reason I want to make a mini-version of an exterior area is so I can stage a HUGE battle (Not kidding, we're talking more than 500 soldier units, along with about 100 ships) without leaving a bunch of markers and things cluttering up the vanilla zone. By staging it in it's own little mini world, the battle can occur in a highly controlled area and when it's done,leave almost nothing in the outside world (Apart from enabling some prevously disabled shipwrecks on the ocean floor in case the player wants to go dumpster diving for loot.)