I use Inkscape 0.92 and frequently use the "resize page to content" tool (see snapshot below) with the same values for the margins. Naturally, I would like to be able to simply set a default value for the margins. Is that somehow possible?
Yes it's possible.
In your Inkscape program directory, find the templates folder, and open the file named default.svg
Go into File > Document properties
Change the margins in the resize page to content section.
Close the document properties
File > Save, and then quit Inkscape
Now when you open Inkscape again, the default template will have the margins you set previously.