<p>I using SharePoint Designer to create a workflow. I'm trying to get at a sub-folder in a document library in the "Define Workflow Lookup" dialog.  There are two issues with this:</p>

<ol>
<li>I can't look up items by URL Path. If I look up by Title, I can
output the URL Path, but selecting by path doesn't work.  What
fields can/can't I use?</li>
<li>I can't get at any sub-folders.  I can get at the top-level folder,
but the sub-folders don't seem to be available. Noticed the same
things is true when looking at the data for a document library in
the "Data Source Library" in Designer.</li>
</ol>

<p>To clarify, the workflow is on a different list, not on the document library.</p>

<p><strong>UPD:</strong> Also, I know how to do this through the object model, the question is how to do it in SharePoint Designer without deploying code to the server.</p>

I really don't have much experience with Sharepoint, but I thought I could at least provide
some
answer - even if it's the wrong one.
From another dev I've spoken to it sounds like it's tough to get into any subfolders, so you might need to look at making your own custom workflow.
Maybe something like LINQ to Sharepoint might be able to help you with actually getting in and enumerating the subfolders and getting to the data that you need?
LINQ to Sharepoint