Has the half-life of an isotope ever been accurately predicted before it could be measured?
A quick perusal of the post-WWII literature (particularly Glenn Seaborg) pulls up papers like The New Element Californium (Atomic Number 98) where comparisons of the observed alpha decay half life are said to be in line with predictions. Such predictions follow along from papers such as Systematics of Alpha-Radioactivity.

So, yes, it would seem that predictions of radioactive half lives were part and parcel of the creation of new elements in the laboratory.