Hm... the modern engravable might be a tag pendant rather than locket (although folks wear those too) and a charm necklace or bracelet offered with a choice of charms would allow to pic the right initials. Just an idea.
Custom work is nice if you want to put the time and money... The task might be a tad tricky unless the folks do not do -from-the-scratch-custom, but rather assemble ready-made letters and call it so
