After playing around with the bookmark saving functionality, I figured out what I was doing wrong. I didn't fully understand the difference between Hostname and Path parameters.
<?xml version="1.0" encoding="UTF-8"?>
<plist version="1.0">
  <dict>
    <key>Protocol</key>
    <string>s3</string>
    <key>Vendor</key>
    <string>s3-role</string>
    <key>Description</key>
    <string>S3 (Credentials from EC2 Instance Metadata)</string>
    <key>Context</key>
    <key>Username Configurable</key>
    <false/>
    <key>Default Nickname</key>
    <string>
S3 (Credentials from EC2 Instance Metadata)</string>
    <key>Password Configurable</key>
    <false/>
    <key>Token Configurable</key>
    <false/>
    <key>Anonymous Configurable</key>
    <false/>
    <key>Default Hostname</key>
<key>Default Path</key>
<string>/bucket-name</string>
  </dict>
</plist>