I have a dialog which contains a pygtk.treeview for listing tasks by priority. Each row has the background colour set based on that priority, so for example the highest prio
You could add a separate pixbuf cell (say, the same size as a small icon) to the far left to indicate selection. Selected rows could fill this with a more "solid" (saturated) version of the colour used for the background. For example. if you use a pink background for high priority, you could use red for the selection indicator. Or you could use an icon.
To implement this with the colour filling method:
You can then color or uncolour the "selection cell" upon selection changes to indicate which row is selected, or display an icon (eg. a stock symbol).
Note that I haven't implemented this, it's just an idea. It departs significantly from the usual GTK selection indication, so (obviously) use your judgement as to whether it's useable.