I decided to use Inkscape, and to treat the plotter head as an engraver. Inkscape has a nice extension called Gcodetools that will allow you to export paths to G-code. I used the text function to type a few simple words in, broke them down into paths, and exported the file. I used the serial terminal I had on my Ubuntu test system to send the file to the TinyG board. After a couple of issues tweaking the feed rates, homing the pen, and inverting the X axis, I was able to get a pretty nice first result!
I still have several small tweaks to do to the system, including tramming, getting the bed support attached to the bed, routing the hold down channels, and bolting/screwing the base to the sides. Right now I'm just using a couple of pipe clamps to hold everything in alignment as I tweaked various things. I'm pretty surprised about the amount of parallelism I have even without tramming - the pen didn't really drift much.
I also don't seem to have any issues with backlash or overheating, at least for now. I rewired all of the steppers to be in parallel mode, which theoretically draws 2.8A, over the 2.5A the tinyG drivers can support. I should be able to manage that using the current pots on the board however, and even in my current configuration I see the benefit. I'm looking to get another power supply from Jameco, which I think will also allow me to run at higher seek rates - right now I'm running about 225 mm/min max.
Before I move on, I'm going to play with this setup for a bit. I believe I can take a photo, run some edge detection, then have Inkscpae build me a vector path to plot the image.