UPDATE:
I also tried mirroring what gets spat out in the source code for the editors built in css style dropdown...
$editor->CustomAddons =
"<br clear='both'/>
<table class='CuteEditorGroupMenu' cellSpacing='0' cellPadding='0' border='0'>
<tr><td class='CuteEditorGroupMenuCell'><nobr>
<span
class='CuteEditorDropDown'
id='CE_Editor1_IDcssclass'
Command='CssClass'
onchange=\"CuteEditor_DropDownCommand(this,'CssClass')\"
RichHideFirstItem='true'
_IsRichDropDown='True'
style='display:inline-block;width:70px;height:20px;'
>
<span val=''
selected='True'
html='Css Class'
txt='Css Class'>
</span>
</span>"
.
"</nobr></td></tr></table>";
... which almost works but screws up the toolbar layout. If I take out the _IsRichDropDown='True' it behaves but is no longer a dropdown menu (It's just a box where the dropdown goes)
Any ideas anyone?