I think this is what you want....
In the tiki-edit_submission.tpl page, change this:
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<textarea class="wikiedit" name="heading" rows="5" cols="80" id='subheading' wrap="virtual"> {$heading|escape} </textarea>
to this:
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<textarea class="wikiedit" name="heading" rows="5" cols="80" id='subheading' wrap="virtual"> {if $heading}{$heading|escape}{else}[ Insert Link Here | Source]{/if} </textarea>
This way, if the page already has a heading, it will be displayed. However, if the page does not yet have a heading, the user will see: Source
-Rick