I found the answer in the Elves source book - the following passage is included under the stats for Elves with notable smithing skills, such as Feanor and Celebrimbor:
Note: MERP rules do not cover alchemy - the creation of enchanted articles - because, in Middle-earth, embedding magical properties into weapons, armor, and other articles is much more than a systematic process. Unique rituals, known only to certain individuals or groups (such as the Gwaith-i-Mirdain), must be used in special places on special materials. Mere cleverness or skill or book-learning does not suffice.
This dodges the question why the game designers didn't include rules for the "unique rituals" required to craft magic items. Game balance issues?