The optimisitic locking is a solid way to approach this. I would not devise a custom/app-specific locking strategy inside the database as you described earlier. That's just prone to more issues, complexity and maintenance.

Regardless of Rdbms vendor locking or your custom locking, holding a lock for 5+ minutes is too long on a busy system. Seems that would lead to transaction blocking or even deadlocking. Good luck with your project.