To answer my question:
There is another simple way to achieve the effect of stroking a dashed outline with line caps:
First, the path is linearized and an two raw offset curves are computed.
The offset curves are stroked with any stroke style (e.g. rounded caps).
Next, the original curve is filled on top of the two offset curves.
This works for outside offset curves. For inside curves I suppose this can be achieved with compositing.