I know this is a super old post but it's one of the first things that comes up when searching for this issue so I'm going to post what I think is the best solution: the Select Curve extension by ThomThom

https://extensions.sketchup.com/pl/content/select-curve-tool

This extension lets you select all lines that are connected by their endpoints with a single click. It's very powerful when combined with FredoCorner, as it easily lets you select an entire edge without selecting every edge that's part of that face. Sometimes you want to round the outside edge of a solid without rounding the inner edges.

Anyway, I hope this helps!
-Heca