Turns out I was wrong. The XML -> SQL is handled by Civ5 itself, not ModBuddy, so we should be able to make or edit XML and LUA based mods without it. I imagine we're going to end up with a modding situation roughly the same as Civ4.
That's correct. It does look as if a lot of the mod functions can be carried out by hand using a text editor on a Mac. Below is my take on how the SDK functions might have to be replaced.
The four components provided by the SDK are:
ModBuddy
The big area where ModBuddy helps is in configuration management. If you build a mod by hand with a text editor, you will also have to make sure that all the mod id, version numbering, files, entry points, actions and compatibility elements are correct in the modinfo file. ModBuddy also provides xml and lua code editors with syntax coloring and code completion etc. This should be able to be done using BBEdit or Xcode, with appropriate plug-ins. And it deals with packaging and submitting your mod to the Mod Hub, but you can still distribute mods independently of the Mod Hub by 7zipping them into a .Civ5Mod file.
Note that you can also include sql text files in your mod, which is potentially very powerful. You can do far more with one line of sql than with a page-full of XML for some tasks. Adding one hit point to every unit in the game is a simple example. It should also be possible to script sql calls in lua.
FireTuner
There is also a debugging facility called FireTuner. It provides a window on the running state of a game, with access to all the sqlite data structures, and event logging. Some of its capabilities are actually available using Firefox sqlite browser plugin, and may also be supported by some Mac tools. Note that sqlite is the part of the HTML5 spec that supports local storage in web browsers, and is also an underlying technology in OS X CoreData.
Nexus
DDS files have to be uncompressed RGB images with alpha channel. In the SDK, these are processed by the Nexus tool to be compatible with the game, which uses a proprietary compression format. I'm not clear on whether it will be possible to create mod Art without Nexus or some Mac equivalent.
WorldBuilder
WorldBuilder will have to be ported, but that's probably no big deal.