Answer: I'd go with the first choice - lime chips or pebbles. The amount of lime leaching from your mulch material would be minute and while it could change the soil's pH over time, I don't think it will be harmful to your Pittosporums. You can test your soil pH now and then test it again in 2-3 years. If you see a marked change, you can counteract the alkalinity with an acidified fertilizer or by sprinkling some sulfur on the soil.
Best wishes with your garden!
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