Can I use metal J-channel and drip caps intended for roofing as my window casing beads?
Galvalume is a trade name and is not the same material as galvanized steel. Englert, a major roofing manufacturer has a good technical description that specifically cautions against placing Galvalume in contact with mortar.

Believe it or not, the entire construction is not comprised of imbeciles, and building codes actually provide sound guidance in regard to what works.