... and I believe the "trick" should work in mobi fine. The following mobi code would give no visual indent, as far as I know:
<p><br>This paragraph has no indent.</p>
Of course, the proper way to have a non-indented paragraph in mobi would be:
<p width="0">This paragraph has no indent.</p>
If a mobi converter does not create the first from the equivalent ePUB, it's a bug (in my opinion).