I don't think the Work Offline button in Explorer is supported anywhere outside of Explorer or duplicated in any form which other apps display (e.g. context menus).
I use this button to quickly open Explorer in the same folder I'm viewing in Opus, which can be useful to quickly access that button or any other rare things which are only present in Explorer:
(How to add buttons from this forum to your toolbars)
<?xml version="1.0"?>
<button display="both">
<label>Open in Explorer</label>
<icon1>#find</icon1>
<function type="normal">
<instruction>cd C:\</instruction>
<instruction>Explorer /e, {sourcepath$}</instruction>
</function>
</button>