The only problem I see with this type of mod is that you are rendering a single ASCII character as a function. Implimentation of this will be harder then you might expect.
Well, Dean's already done it. Here is some source code for Textile 1.
Dean is very busy with Textpattern, but he told me he'd do some work on Textile 2, including releasing a PunBB mod, as soon as the work on Textpattern calms down a bit. So it's coming.