Don’t know the answer, but here are some details: You can use databinding to add actual value to placeholder elements. LibreOffice only displays the databindable element’s placeholder text.
Here is an article about how to do it.
And here is a sample docx: ContentControls.docx.odt (it is a docx, ignore the extension. Just can’t upload files with docx extension for some reason) that contains a content control. Word shows “data from databinding”, as set in the data-source xml, but libreoffice (checked with 4.0.0.3) only shows the placeholder text.
The data used for databinding, if someone doesn’t have that Content Control Toolkit:
<root>
<something>data from databinding</something>
</root>
The element in the document:
<w:sdt>
<w:sdtPr>
<w:dataBinding w:xpath="/root[1]/something[1]" w:storeItemID="{4eca48a1-5a98-425b-a2ec-98e9f57ae606}" />
<w:alias w:val="titleOfFirstElement" />
<w:tag w:val="tagOfFirstElement" />
<w:id w:val="-1614272614" />
<w:placeholder>
<w:docPart w:val="DefaultPlaceholder_1082065158" />
</w:placeholder>
</w:sdtPr>
<w:sdtContent>
<w:p w:rsidR="00570B6E" w:rsidRDefault="00570B6E">
<w:r>
<w:t>placeholder text</w:t>
</w:r>
</w:p>
</w:sdtContent>
</w:sdt>