In this note, we demonstrate the high-temperature growth of GaN on ScAlMgO4 substrates by metalorganic vapor phase epitaxy when a thin Al film is deposited ex situ on the ScAlMgO4 surface, prior to GaN growth. Mirror-like high-quality GaN epitaxial layers were obtained when N-2 was used as carrier gas during the reactor temperature ramp-up preceding GaN growth, leading to a higher GaN quality compared to direct growth on ScAlMgO4 using a trimethylaluminium preflow. This opens a pathway for high-temperature GaN growth on ScAlMgO4 when an Al precursor line is not present.