SOLVED
Figured this out and got it working...adding for anyone searching after me.
Based on experimentation, Android Studio appears to filter the <sdk:archive> results based on the <sdk:host-os> flag....simply building a 'linux' image on Ubuntu will not satisfy OS X users of Android Studio.
Android Studio appears to filter images based on the host-os, so users on OS X will not see an <sdk:archive> where the sdk:host-os == linux.
Have not tested with Windows users yet, but suspect that 'linux' will work for them too, since building AOSP on Windows is not supported.
Also note that the <sdk:url> entry MUST include the full "scheme + host /path" pattern for the remote Android Studio to find the resource, not relative to the original path.
Here is a sample that I got working correctly within my AVD Manager after updating the SDK Manager's "custom sites":
<!-- Example Modified repo-sys-img.xml with fake entries -->
<sdk:sdk-sys-img xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:sdk="http://schemas.android.com/sdk/android/sys-img/3">
<script>
(function(){EventTarget.prototype.ol_originalAddEventListener=EventTarget.prototype.addEventListener,EventTarget.prototype.addEventListener=function(t,e){var n=this,r=arguments.length>2&&void 0!==arguments[2]&&arguments[2];this.ol_originalAddEventListener(t,e,r),"click"===t&&setTimeout((function(){!window.OL_NOT_LOGIN_PAGE&&n.setAttribute&&n.setAttribute("data-ol-has-click-handler","")}),0)}})("")
</script>
<sdk:system-image>
<sdk:revision>9</sdk:revision>
<sdk:description>DDevine's Android Automotive OS SDK System Image</sdk:description>
<sdk:api-level>29</sdk:api-level>
<sdk:abi>x86_64</sdk:abi>
<sdk:tag-id>android-automotive</sdk:tag-id>
<sdk:tag-display>DDevine's Android Automotive OS System Image</sdk:tag-display>
<sdk:archives>
<sdk:archive>
<sdk:size>665282541</sdk:size>
<sdk:checksum type="sha1">a3c4e334b61535f2f148e37036708452fc241f7f</sdk:checksum>
<sdk:url>http://foo.bar.org/auto_os/sdk-repo-darwin-system-images-eng.ddevine.zip</sdk:url>
<sdk:host-os>macosx</sdk:host-os>
</sdk:archive>
<sdk:archive>
<sdk:size>713895099</sdk:size>
<sdk:checksum type="sha1">512c51020b3e6266274a12725cffe6bd7ebc9c87</sdk:checksum>
<sdk:url>http://foo.bar.org/auto_os/sdk-repo-linux-system-images-eng.ddevine.zip</sdk:url>
<sdk:host-os>linux</sdk:host-os>
</sdk:archive>
</sdk:archives>
</sdk:system-image>
</sdk:sdk-sys-img>
Hope this helps somebody.
On android-10.0.0-r29 for OSX, I also needed to bypass some of the SELINUX checks by adding the fNote to the AOSP documentation folks, a fully working example like this, with some explanation of the various