Wow! I got in the rhythm of working on the project that involves this display and completely forgot about this thread. I did get it running pretty well in landscape mode without much difficulty. I'll post code when I have access to my primary computer again. Until then, I'll explain what I did.
I modified the lowest level stuff possible - drawFastLine(), drawPixel(), fillRect(), and maybe drawChar(). I'll have to go back through and look. Prettymuch anything that calls setAddrWindow() and starts writing via SPI needs modification.
All I did was change up the logic so that the lower left and upper right corner of any "landscape mode" operations you do would be the ones fed to the setAddrWindow() function, rather than the upper right and lower left as would be in portrait mode. Also, any height/width checks need reversal.
Two additional things of note. One, the display seems to work fine at SPI 9MHz. Two, fillRect() is bugged. Any rect you draw will be one pixel too long on one end, so it will render improperly. So a 10x10 square will actually be 11px long in one dimension, so you'll get an 11x9 rectangle with a little pixel tumor where the 100th pixel went. Discovered this and fixed it when I was duct taping the library.