While I have focus groups right there in my page relative to an element inside and RG, when I add a new focus group, the new focus group is not able to find any element inside an RG in the drop down to set the relative element.
It seems I’ve been able to do that, and I managed to figure this out again a moment ago:
The way I did it was to make a reusable element with the focus group in it then once the re-usable element is in the repeating group, to detach the re-usable element, which makes the focus group and other elements of the previously re-usable element behave like local element, with the exception that the focus group is still able to consume the reference to the element inside the repeating group.
It also has the bonus of letting the focus group sit close to the element it is related to on the element tree rather than being listed in overlays.