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I think so because of the [name_m]Jacques[/name_m] connection. The Wikipedia article for [name_m]Jack[/name_m] lists [name_u]James[/name_u] as a possible source for [name_m]Jack[/name_m], citing the same connection, so you wouldn’t be the first to do it.
I think [name_m]Jack[/name_m] for [name_u]James[/name_u] is totally fine. I tend to assume that [name_m]Jack[/name_m] is its own name nowadays, rather than a nickname, though!
My nephew is [name_u]James[/name_u] and goes exclusively by [name_m]Jack[/name_m]. It’s never caused a single problem and 7 years later it seems so natural.
[name_m]Jack[/name_m] works for [name_u]James[/name_u]! For sure.
I also think that [name_m]Jack[/name_m] works fine as a nickname for [name_u]James[/name_u]. People use [name_m]Jack[/name_m] as a nickname for [name_m]John[/name_m] all the time, so I don’t see why [name_u]James[/name_u] would be any different.
[name_m]Jack[/name_m] is a great nickname for [name_u]James[/name_u]! I love the name [name_u]James[/name_u]!
Yes I think it works, particularly because of the [name_m]Jacques[/name_m] connection, but fine anyway.