Just came across Halachipedia's bit on this (which perhaps is more of aan ex post-facto limud zechus facto limud zechus on an established custom rather than an a priori, l'chatchilal'chatchila leniency):
...parents blessing their children and Rabbis blessing their students, may put both of their hands on their heads while reciting the blessing, since the only prohibition is to make the blessing with the intention of fulfilling the mitzvah of the commandment, and to do it like the cohen.
Yalkut Yosef, Tefillah Volume 1, page 296. See Beiur Halacha 128:1 s.v. DeZar who writes that the minhag of Yisraelim to bless other Jews with the words of Birkat cohanim is either based on the fact that perhaps there's only a prohibition if they raise their hands like cohanim or that they have kavana not to fulfill the mitzvah of Birkat cohanim.