Many thanks for your feedback, Gents. I suppose I'd always assumed spot drills were not really usable "sideways", but clearly I'm missing a trick based on what you're saying. I guess they are quite rigid cutters after all.

And the best part is that I have a small selection of them already, uncoated, which I can try out. If they work, I need spend nothing extra on tooling for this task - how often can you say that?!

I really appreciate the feedback, and will have a play when I get a chance.