I'd like to plot Bollinger Bands on some of the existing studies such as MFI and RSI but haven't seen how to do this yet. I'm assuming that I'll be stuck writing a custom indicator via ThinkScript but haven't seen any examples of something like this when searching either.

Has anyone here done this and might be able to point to some examples?

There's free study on Eric's blog that seems to be very similar of what you want....
it's so easy to rewrite it with just any indicator....for example with standart Wilder's RSI
Here's how it will look like


And you can get it right from download area.....Enjoy