Just heard from Yahoo Tech Support. Not good news. Here's the bulk of their reply:
"...the name .htaccess refers to the main configuration file for
Apache, a popular open-source HTTP server. The .htaccess file can be
used to create custom error pages and web page redirects, set up
password protection, enable SSI, and more.
Yahoo! does not currently allow you to upload .htaccess files to your
account and currently there is no workaround for it. For other
customization options, we recommend the Custom Error Pages and
password-protection features available in your Web Hosting Control
Panel.
Thank you for your understanding in this regard."
I'm assuming this means I cannot use Zen Cart with my Yahoo accounts. Which is too bad because I did some research and it appears that I get far more disk space, bandwidth, statistical reports, and a lot of other stuff from Yahoo Web Hosting, than I can get at the featured Zen-certified sites within the same price range.
Any advice at this point would be very much appreciated. Please!