Analysis

I have to admit this was nothing I anticipated. The circles and squares are late additions, and didn't make it to the final release of the sidebar. As such, the mechanism is (still) internal and needs to be overwritten explicitly. The culprit is in the following code:

To be honest, I don't exactly know how I came up with this form of template and why I used exactly these lengths. I guess it felt right at the time.

This is how the skill is defined and later used in the sidebar:

So you could indeed redefine these commands within the sidebar, since the original definition will be restored at the end:

Current Workarounds

This is not as straightforward as for the main body, because there is (currently) no accessible template for a single (skill) line. However, you can define it yourself to whatever you want with quite low-level commands. A very simple reimplementation would be:

  \begin{polycvsidebar}%
% [...] 
  \renewcommand{\cvskill}[2]{%
    \parbox{1.0\linewidth}{#1\hfill\levelbarsquares[0.4\linewidth]{#2}}%
  }
% [...]
  \end{polycvsidebar}

For long descriptions this might look odd and brake. More complicated and possibly a little bit more robust:

  \begin{polycvsidebar}%
% [...] 
  \renewcommand{\cvskill}[2]{%
    \parbox{0.6\linewidth}{#1}\hfill%
    \parbox{0.3\linewidth}{%
      \levelbarsquares[\linewidth]{#2}%
    }%
  }
% [...]
  \end{polycvsidebar}

This would allow for different implementations of skill and language.

Alternatively, you can change the internal definition of the command that will be loaded automatically. This will simultaneously effect cvskill and cvlanguage. So keeping it close to the original definition, putting the following code into the preamble will change the progress indicator to circles:

\makeatletter
\renewcommand\polycv@sidebar@skill[3][\polycvhintcolsep]{%
  \parbox{1.0\linewidth}{%
    \hspace{#1}%
    \parbox[b]{2\linewidth/3-4#1}{#2}%
    \hspace{#1}\hfill%
    \parbox[b]{\linewidth/3}{%
      \levelbarcircles[\linewidth]{#3}%
      }%
    \vspace{0.2\baselineskip}}%
}
\makeatother

Further considerations and development

I guess the easiest to make this more customisable is to mirror what is happening in the main environment.
That probably means to define alternative templates for a skill line, which can be redefined and would be loaded automatically. Something like \cvsbskill and \cvsblanguage and a more generic version of a line. I have not yet made up my mind and ideas will be welcome.

Since these are not really breaking changes, but additions and internal redefinitions, I assume they can go into a v1.1.

The change is now live in the development version. This should be as easy as for the main body:
(The workaround line is carelessly left in from the previous attempt.)

Please test this and let me know what you think.

This workaround is working fine. It does what it's supposed to do.

In the same style I also added \newcommand{\sidebarline}[1]{\sbline{#1}{}} neglecting the rating bars for lines in which they are not necessary or wanted (e.g. certificates, hobbies or whatever floats your boat).

This is already the new implementation, no longer a workaround (which was to redefine the internal commands).

I'm not sure, beyond what I have already tested, there is anything left to do. I guess I could escalate this to a release candidate status soon then.