For those brave enough to edit raw files:
exiftool.exe "-usercomment<filename" -@ {allfilepath|filem}
Button as XML
<?xml version="1.0"?>
<button backcol="none" display="both" label_pos="right" textcol="none">
<label>Copy Filename to Comment</label>
<icon1>#newcommand</icon1>
<function type="batch">
<instruction>@nodeselect</instruction>
<instruction />
<instruction>exiftool.exe "-usercomment<filename" -@ {allfilepath|filem}</instruction>
<instruction />
<instruction>pause</instruction>
</function>
</button>