Hi. Could we use the word "beloved" to address someone in a letter? Thank you for your help in advance.

Dear beloved friends
Dear beloved John Doe
Generally we don't include beloved when addressing someone in a letter. I could imagine Dear beloved friends being used in rare circumstances, perhaps by a preacher writing to his congregation. Dear beloved John Doe sounds very odd to me. I would be surprised to see it used.
'Beloved' is like an extreme form of 'dear', so using both together is redundant.

Dear friends normal and common

Beloved friends Uncommon, except eg when a priest is speaking in Church.