You could handle this by defining a style for your list elements.
Hopefully Colum won't mind my offering a slight twist on what he offered.
I believe once you define a Multi-Level list style you will find it in your CSS. My thinking here is that you might be able to influence it on a more granular level by hand editing the CSS file using Windows Notepad or perhaps a third party CSS editing tool.
if I assign a paragraph style to the list level, then any indention is simply ignored. The indention is hard-coded in the HTML in steps of 10px and overrides any CSS-definition of the paragraph style.
Or did I misunderstand you?