Hey guys, thanks for the info. I got it figured out. For anyone who happens across this, what I had to do (using the tools I prefer) was:
1. Get the resource out. In the case of the wide format track slider for the Inspector/Console view, it's bitmap resource 2671 (intuitive enough, right?).
2. Edit the resource, or use it as a model for size and background color for your new creation. You want to make sure to center your stuff in the box horizontally and make sure you know where the vertical middle marker needs to be. Aside from that, it seems pretty flexible. I use Xara Xtreme for this, as it's an awesome app that can so some cool vector stuff for these types of graphics really fast. Unfortunately, it doesn't save to 32 bit bmp.
2a. When I create this, I use layers - the bottom layer is the grey rectangle. The top layer is the actual item I want to see.
3. Export to PSD so they layers come across.
4. Open my PSD in GIMP, go to the layers pallette and select the bottom grey box layer, choose to lock alpha transparency to this layer (don't know if you need to do this or not) and change it's opacity to 0% (fully transparent).
5. Save the image as bmp, choosing Merge, and going to the advanced options along the way and making sure to export in 32bit A8R8G8B8 mode.
6. Reintegrate the bmp that gets saved to the proper (2671 in this case) leaf under the bitmap tree.
Launch Sonar - nice...it works.
Here's my screwing around "ever so popular orange gel buttons" made faders. I have some interesting ideas here, but I will have to find the time to dig around the resources to get to the stuff I'd like to change: