While adding some features to the toolkit so that SNSes can manage neurons from other SNSes through proposals I ended up in a discussion to do the something similar for ICP neurons, while exploring the possibilities in ran into the following issue;
In relation to SNS neurons, ICP neurons are not public by default and can only be fetched based on the caller. Because the SNS governance canister is be the owner of these neurons it’s not possible to retrieve them.
There was an big discussion going on regarding public ICP neurons but i didn’t spot any mention of SNSes that own ICP neurons. For sake of transparency I would expect that these SNS controlled ICP neurons are public.
Did notice that @GOLDDAO 
has a custom implementation for this by creating a canister that also holds references to their ICP neurons (creating semi-transparency themselves).
If i’m off with my findings, please correct me, if not, I think the least invasive approach to make this happen is by adding a call to the SNS governance canister that fetches the neurons from the NNS governance canister by caller.