After looking through the actions and verbs I don't see a way to do this. 

In the actions I see the following:

transform-remove    :  Remove any transforms from selected objects
transform-rotate    :  Rotate selected objects by degrees
transform-scale     :  Scale selected objects by scale factor
transform-translate :  Translate selected objects (dx,dy)

But no skew.

Is there a way to do this?

Why do I want to do this? I want to insert text into an isometric template programmatically and so need to skew it to have it look right.
 

I do not think it is possible.

You can easily replace the transform attribute, however is it a replacement not additive. Unless you use an external script to retrieve the current transform then combine them. 

This will add a skew, but it is destructive.

inkscape --batch-process --actions="select-all;object-set-attribute:transform,skewX(30);export-filename:skewed.svg;export-do;" testfile.svg

If you write your own python extension, they can be accessed from the command line just like any other verb.

Thanks for the answer. I am going to have to play around a bit to see if I can achieve the result I am looking for.

I guess the alternative would be to do a perspective transformation of my text object using my desired isometric shape. Do you think that would be possible from the command line?