AFAIK, there's no way to do what you want using widgets. The way the links to pages and sub pages work in Regulus is hardcoded in the theme itself.
One way that you could create a widget with links to all top level pages is:
Using a text widget, do something like this:
<ul>
<li>Page 1</li>
<li>Page 2</li>
<li>Page 3</li>
<li>etc.</li>
</ul>
However, there's a page widget for this already, so the above sample would be just redundant. BUT you could do this to list your sub pages. However, they would be displayed on every page/post of your blog not on the specific page they belong to.
Another option would be to list your pages and sub pages like this:
Page 1
sub page 1
sub page 2
sub page 3
Page 2
sub page 1
sub page 2
sub page 3
etc.
If you consider doing something like the above, then in a text widget you could write the code like this:
<ul>
<li>
Page 1
<ul>
<li>sub page 1</li>
<li>sub page 2</li>
<li>sub page 3</li>
</ul>
</li>
<li>
Page 2
<ul>
<li>sub page 1</li>
<li>sub page 2</li>
<li>sub page 3</li>
</ul>
</li>
</ul>
HTH