In An Tuiseal Gairmeach The Vocative Case, on page 22 of my copy of Gael Linn's Gaeilge agus Fáilte, it explains the use of the Vocative, and ends with "This rule does not apply to names that are not in the Irish language e.g. Conas tá tú Simone?" Does this mean then that the made-up names at these postings would not get the séimhiú when being addressed in the vocative?

I think it usually depends on the writers inclination, that there's no hard and fast rule to it. I've seen things like a Jhason before, usually people just play with the language as they see fit. Words in Irish but that aren't names, eg mo cheannsa, can be vocative-ised.