In theory you can't, but isn't that exactly what happens in many non-union workplaces? Suppose your daughter grows up and wants to become a journalist. Say, she wants to write for the Nation. Even if you don't lift a finger for her, won't she have a better chance than a daughter with a non-journalist parent, not to mention a daughter of a blue-collar worker?
My father was a steelworker (now retired), and I haven't met many grad students & professors who hail from similar blue-collar backgrounds. (Mike Yates seems an exception.) I could be proven wrong by empirical evidence to the contrary, but if not exactly the same jobs, most children seem to 'inherit' jobs comparable to their parents' (or at least the kind of education & experience that would make them likely candidates for them). Isn't that how stratification and segmentation of the working class get reproduced?
So, it seems this question of job inheritance isn't limited to craft unions at all, and at all levels, union or non-union, women and racial minorities are at a disadvantage. Considering this, I think having a union beats not having a union at all. In unionized workplaces, at least there is a chance to fight to do away with the kind of nepotism and informal exclusion that you are concerned about (as IATSE has to a certain extent).
Isn't the proportion of writers who are women and/or racial minorities lower in print journalism (including liberal or left journalism) than in IATSE?
Yoshie