Hi _luke,
try the following: Let the submenu be created by a Ditto call, not a Wayfinder call.
Create a chunk named submenuchunk containing this:
<ul class="submenu">
[!Ditto? &parents=`[*id*]` &tpl=`yourtemplate` &display=`99` ... !]
</ul>
and put the Ditto call into the content area of the document as {{submenuchunk }}.
&parents=`[*id*]` lets the submenu list only the children of the actual document;
yourtemplate is the chunk where you define the output for the submenu item.
A high value for &display is needed to guarantee that all subitems are listed.
(If you omit the parameter &display, its value is set to `3` by default - and you may
wonder why you won’t get all of your subitems! - I lost some time on that...)