Adam, Thanks for your efforts, but I had to customize a solution.
I'll post it here for anyone else in my situation.
You don't need to address it any further, I'm satisfied with where I'm at.

Since we'll be dealing with nearly 20,000 images, the database option is not an option.
The UNC path is also not an option in our environment.

1. ~ I set "AllowUpload", "AllowRename", "AllowDelete" etc. all to false. Any Allow* that would look for that path.

2. ~ (optional) Then in the Dialogs, every Dialog aspx file that had "fileUpload" as an id, I set to hide. Changing them to 'false' doesn't hide them, it only disables the Browse and Upload buttons, which I thought was a little ugly.

This had to be done in two ways - For the ones that had "runat=server" in the fieldset with the id="fileUpload", I added this line above it (I'm showing the actual fieldset line for explanation purposes.)

  <% if (!secset.AllowUpload) { fieldsetUpload.Visible = false; } %>
  <fieldset runat="server" id="fieldsetUpload">

For the ones that had no runat="server", I added an if clause.

  <fieldset id="fieldsetUpload" <% if (secset.AllowUpload) { %>style="display:none;"<% } %> >
 
3. ~ I then modified the dialogs (like 15 of them, I think) so that when the fieldset was hidden, there was a link to "Upload Images" - a page I created cased upon the next step...

4. ~ That link led to a script I got from 15seconds.com called "Creating a File-Upload User Control with ASP.NET"

And, voila, we have a winner.