Not through the Caption dialog, but you can accomplish this with a table.
Insert the caption above or below the equation (in a separate paragraph).
Then insert a two-column borderless table and put the equation in one cell
and copy/paste the caption into the other.
If you won't need to cross-reference the equation, you can use tabs instead,
a center tab at the center of the line for the equation and a right-aligned
tab at the right margin for the caption. If you cross-reference this
caption, however, because it is in the same paragraph with the equation, the
cross-reference will pick up the equation as well. When you use a table, you
have two separate paragraphs, so this is not a problem.
