take selected text, indented by spaces, and convert them to divs.
[...] I may have to build my own script!
You're familiar with the "File > New From Template..." option, I hope? And the multitude of snippets and macros already extant in the HTML bundle?
Try the 'div' tab-trigger some time (from a new HTML 4.01 template, which you can edit to suit your tastes), I think you'll find yourself struggling to justify such a hairy script as a "MarkupMaker" (there's always Markdown, if you don't want to write in HTML but desire HTML output).
Unless you've got reams of existing, single-space indented (?!) hierarchical content a-waiting for the generator of your dreams, I think you'd be better off learning how to better employ the wonderful tools you already have in TextMate. Ciarán Walsh's blog [1] has a number of bite-sized, "bet you never knew you could do this" bundle tutorials; there are others but a Google away.
HTH, Daniel
[1] http://ciaranwal.sh/ (the coolest domain + TLD combo ever)