What you can do, is just make the heading in the text like you want to, so 'HEADING'. Create the epub. Now you have to edit the epub. Download the TweakEpub utility and run it. Open the created epub and edit the toc.ncx file.
In there, I would expect something like:
Code:
<navPoint id="chapter01" playorder="2">
<navLabel>
<text>HEADING</text>
</navLabel>
<content src="chapter01.xhtml">
</navPoint>
Of course the names of the id, playorder and filename will be different. Change the text of 'HEADING' here in 'Heading' and save. Save and close TweakEpub. If you now open the ePub, it should be as you would like.
Keep in mind, that if you open the book some editors (like Sigil), it will change back. I think that in the next version of Sigil this will be possible.