Thanks for the suggestions @David and @Skillmon. I think I achieved what I imagined with this:
\documentclass[10pt, a4paper, oneside]{memoir}
\usepackage{graphicx}
\usepackage{lipsum}
\usepackage[pass, showframe]{geometry}
\begin{document}
\marginpar{
\rotatebox{45}{%
\hspace{-60pt}
\begin{minipage}{1cm}
\parshape=6
60pt 89.0954pt
48pt 89.0954pt
36pt 89.0954pt
24pt 89.0954pt
12pt 89.0954pt
0pt 89.0954pt
\noindent
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed do
eiusmod tempor incididunt ut labore et