Okay, I figured out how to do this, and in the process uncovered a bug in mojoPortal. The technique is based on this thread from 2010. On ScriptLoader, you can pass custom jQuery Accordion startup parameters. So ultimately the way to do this will be to have a ScriptLoader in your skin's Layout.Master with paramters like this:
<portal:ScriptLoader ID="ScriptLoader1" runat="server" IncludeSizzle="true" JQueryAccordionConfig="{active:'false',collapsible:'true',fx:{opacity:'toggle',duration:'fast'}}" />
This brings me to the bug. Joe, Scriptloader.cs, line 853, looks like this:
initAutoScript.Append(" $('div.mojo-accordion').accordion(" + jQueryTabConfig + ");");
It should look like this:
initAutoScript.Append(" $('div.mojo-accordion').accordion(" + jQueryAccordionConfig + ");");
So to workaround the bug until Joe has a new release of mojoPortal ready, you can set your ScriptLoader line like this. I'm not sure how this will impact jQuery Tabs widgets, if you have any of those, so YMMV.
<portal:ScriptLoader ID="ScriptLoader1" runat="server" IncludeSizzle="true" JQueryTabConfig="{active:'false',collapsible:'true',fx:{opacity:'toggle',duration:'fast'}}" />
Also, note that this solution is "skin wide", so any page using the skin will start with collapsed jQuery Accordions. So if you want it only on one page, you might want to make a duplicate skin and assign it to just that page.
Ultimately the solution might be to have a new built-in Content Template for "jQuery Accordion Collapsed". This would be a feature request to Joe, or if another developer wants to add the Content Template and contribute it to the project, changes would need to be made in SiteUtils.cs and ScriptLoader.cs. Don't forget to submit a signed Contributor Agreement with your changes.
Jamie