A renderer ain't nothing but a class :)

GtkCellRenderer — An object for rendering a single cell 
GtkCellEditable — Interface for widgets that can be used for editing c
+ells
GtkCellRendererAccel — Renders a keyboard accelerator in a cell
GtkCellRendererCombo — Renders a combobox in a cell
GtkCellRendererPixbuf — Renders a pixbuf in a cell
GtkCellRendererProgress — Renders numbers as progress bars
GtkCellRendererSpin — Renders a spin button in a cell
GtkCellRendererText — Renders text in a cell
GtkCellRendererToggle — Renders a toggle button in a cell
GtkCellRendererSpinner — Renders a spinning animation in a cell
[download]

https://developer.gnome.org/gtk3/stable/GtkCellRenderer.html

https://github.com/GNOME/gtk/blob/mainline/gtk/gtkcellrendererpixbuf.c

https://github.com/GNOME/gtk/blob/mainline/gtk/gtkcellrenderertext.c

this suggests the approach could work even though its not common custom gtk3 cellrenderer

https://stackoverflow.com/questions/14998298/how-to-write-custom-gtk-cellrenderer-in-python-and-gtk-3