Are there any commands from the verb or action list that would let me modify the start/end angles of an ellipse's arc?  I'm looking through them/searching online about it and am having trouble finding any answers.

The short answer is no.

Apart from transforms.

There no way to edit the properties of objects from command line.

It is possible to call extensions from the command line, including custom extensions - but parameters cannot be sent to extensions from the command line.

As far as I am aware, incomplete ellipses are not not part of the svg specification.

Inkscape uses the path element and the sodipodi namespace attributes to the set the shape. I

<path
       style="fill:#ff0000;fill-opacity:0.639216;stroke:#ffaaaa;stroke-width:1;-inkscape-stroke:none;stop-color:#000000"
       id="path846"
       sodipodi:type="arc"
       sodipodi:cx="83.611023"
       sodipodi:cy="117.19216"
       sodipodi:rx="36.364555"
       sodipodi:ry="30.347467"
       sodipodi:start="0.3"
       sodipodi:end="0"
       sodipodi:arc-type="slice"
       d="M 118.35141,126.16045 A 36.364555,30.347467 0 0 1 75.497859,146.77469 36.364555,30.347467 0 0 1 47.654802,112.65709 36.364555,30.347467 0 0 1 86.335808,86.930006 36.364555,30.347467 0 0 1 119.97558,117.19216 H 83.611023 Z" />

If you modify the sodipodi:rx and sodipodi:ry attributes between 0 and 2 * pi 

Inkscape will accept that as valid. The path 'd' attribute is used by standard svg viewers to show the arc. Inkscape does not use it if the sodipodi attributes are present, but will regenerate it if it detects other changes in the document.

So you could add your own sodipodi based ellipse based element and insert it, or convert an existing normal ellipse.

All of this would require scripting.

Thank you @inklinea,  I'll have to look into this when I have more time.  I appreciate your in-depth explanation of a possible solution!