Thanks! With a little bit of tweaking I got it mostly working! I've attached a link to the docset file.
However, if you'd like to produce your own docset, here's what you do:
1) Download the two files above
2) Download doxygen
3) Open the Doxyfile in a text editor and change the INPUT to point to your local copy of TK2DROOT. (e.g. if you put the doxygen files in the same directory as your TK2DROOT you'd change INPUT to look like this:
INPUT = ./TK2DROOT/tk2d \
./TK2DROOT/tk2dUI \
./TK2DROOT/tk2dTileMap \
./DoxygenData.cs
4) Also change "GENERATE_DOCSET = NO" to "GENERATE_DOCSET = YES"
5) Create doxygen_footer.html and doxygen_header.html in the same directory as your doxyfile. I copied the code from the header and footer divs from the current online documentation, and tweaked it a bit based on how it turned out.
6) Download the doxygen_stylesheet.css from the current online documentation (view source on the current documentation and find the link to the stylesheet and download it).
7) Run "doxygen Doxyfile" from the terminal in the directory where the Doxyfile exists.
This will have generated a folder called "Generated" and a directory inside there called "html". Navigate inside the html directory with the terminal. Run "make" inside that directory.
9) You should now have a file called "org.doxygen.Project.docset" in the html directory. Open Dash and add this docset.
That should be it! Thanks again, unikron, for hooking me up with those files.