Success!!! From this thread: OpenFL 7 AssetLibrary
I changed my project.xml file to use:
<library id="assetlib2" type="gzip" preload="true" />
<assets path="embeds/models" library="assetlib2" />
Then, in the hx file I used:
Asset3DLibrary.loadData(Assets.getBytes("assetlib2:embeds/models/cover.3DS"));
MIME types for 3DS and pak had to be added to the server, but we’re in business! I hope this helps someone else.