Rotated characters with transparency

I'm trying to make a circular watch face with the numbers angled towards the centre. To do this I draw each number in turn into an array buffer and save them as individual images so they can subsequently be drawn with appropriate rotation.

The images are created using drawString with the "solid" parameter set to 0, which I hoped means empty pixels would remain transparent. They are saved with asImage("object") but the resulting image has no transparency information and the rotated numbers appear on an opaque background. To clear that I have add a "transparent" field and replace the buffer with a copy stored in a Uint8array.

This works but it seems a really clunky process. Is there a better way to rotate text characters with a transparent background? Thanks.