Hmmm. No, onCanEditGroup won't work, if you are needing to process each repeat individually, as it only gets called once per group, not for each repeat.
I thought we had another candidate in onElementCanUse, which gets called from canUse() in the element model, which gets called from the right place in getGroupView(), where the repeats are being processed.
BUT ... by that point, canUse() has already been called on all the elements, so the access mask is already set and cached, so canEdit() just returns the cached value, and doesn't re-run the onElementCanUse plugins. Which is not something I want to do an end run round, as canEdit() gets called a *lot*, and running plugins from it every time would be a massive performance hit.
I spent about 45 minutes noodling around stepping though code, and I really can't find a hook anywhere that works for what you want to do. I suspect the only way would be to add another plugin hook and fire it seperately around line 4832 in the form model:
https://github.com/Fabrik/fabrik/blob/master/components/com_fabrik/models/form.php#L4830
But of course that's kind of outside the scope of subscription support, and I'd have to do some exploratory surgery to see if changing the access control on it at that point would have the desired effect.
-- hugh