As someone very familiar with UDK, and an ex- Environment artist/ Level designer for Chivalry (I made Arena) I can tell you UDK is nothing like Hammer and even if the Devs release an SDK, it will not be as robust in creating
custom maps as you might think it.
The reason for that is not the Dev’s fault, and surely you are correct about the scripting instructions needed to make a gametype work and players spawn.
Source Engine uses mostly brushes, which are shapes inside the editor, and custom models are used sparingly. You can get away with creating a layout using these brushes in the editor and texturing them as walls and floors ect. This is how most source games are built. The engine prefers this method.
UDK (Unreal 3) moves away from that approach in that brushes are used sparingly and in most cases not at all. If i recall, I dont think there is one Chivalry map that uses brushes other than to block out an idea for a level or such, and that goes for just about any Unreal game.
I can also tell you Unreal 3 does not like brushes. They take a long time to render, and are very un-intuitive to use compared to Hammer. You will not see your updates in realtime for example, and its hard to tell what exact units and sizes you are getting with them. Frankly, they are a nightmare to work with.
Modeling Architecture is the main focus here, and creating complex material shaders to go with them. Sure, there are elements of the content created that can be turned into a level, but without the knowledge and skills to create new content yourself, you will be heavly restricted in what you are able to accomplish as anything architecture related tends to be very map specific.
You could get away with making a map that relys heavly on terrain elements though, and throw in a few simple exterior meshes to go with it. Interior stuff is alot more complicated as its all designed to fit together like lego’s, and is not always backwards compatible with meshes from other levels and so on.
Im not sure what your ambitions are, but in my personal opinion it would be very hard to create something new and compelling for people to get excited about without extensive modeling and texturing experience, as well as fluency with UDK.
Dont let that stop you though. You can still accomplish alot by learning the tools. I would start with learning the free UDK editor and using there pre made assets to get an idea of what a Chivalry SDK would be like. It would be a very similiar experience.