How to set default values for custom attribues in composer ?

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I'm using composer with page types with custom attributes. I set up composer programmatically (thanks to the latest addition to the docs !).
However, I would like to set default values. Some attributes would have literal static values ("0"), but other are calculated on the fly (for instance current date + 60 days).
I tried using the on_page_add event, but it seems that whatever values I set do not stick and I get nothing in composer - I guess the event is firing too early, before the attributes are linked to the page ?
Any other way I could do this ?