hav_ngs_ru

am I only one who has such trouble? )

yes, like this.
to be correct, I meant a function that adds standart wp meta to post AND saves mf relationship, because if I use mf, all add/upd/del meta operations should respect mf relationship.

add_post_meta_mf(post_type,field_name,value, group_index*,field_index*)

or maybe you could hook add/upd/del meta operations, and update mf relationship from here (both for admin-panel updates and for user-called add_post_meta), without adding new functionality, just using existing for both cases.

well, that seems sanely.
but in the abstract it still seems to me not good practice to increment id with every update. even unsigned bigint is not endless.