I've always sort of thought (or justified, to be honest) of these mock-crosses as depending on the bearer.  Just as, later in the series, a real crucifix wielded by a non-believer proves harmless, a pseudo-cross wielded by one with sufficient faith (and sufficient belief that what he was holding counted as a real cross) becomes a true crucifix in that bearer's hands. 
       A justification, to be sure, but has anybody ever seemed more committed or certain of his cause than Peter Cushing's Van Helsing?  Works for me, anyway.