What format does OpenOffice Equation Editor use?
LibreOffice Math (and OpenOffice Math) both use MathML to represent mathematical markup.
Just save any equation using Math and simply unpack it (it is ZIP packed). For example if you create famous [E=mc²] and save it, then in content.xml inside your .odt file you will find something like this:
<?xml version="1.0" encoding="UTF-8"?> <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics> <mfenced open="[" close="]"><mrow> <mi>E</mi><mo stretchy="false">=</mo><msup><mi mathvariant="italic">mc</mi><mn>2</mn></msup> </mrow></mfenced> <annotation encoding="StarMath 5.0">left[ E=mc^2 right]</annotation> </semantics> </math> As you see, Math stores equation in own format annotation section, but you can use plain MathML as input format.
… command-line converters that will process this format
I believe, you can use Python API for Libre (Open) Office.
You can put your own equation in annotation (in StarMath's format). Looks like Math builds equation based on how it is represented in annotation and the rest of MathML file is just for representation/compatibility.
If MathML doesn't look like made from StartMath it will propose to "repair" the file and it will work just fine after this.