HIVE

For a game handeled pos you need a version that has frame child api. Then you walk down the children of ("MultiboardListContainer", 0) and place your sprite frame relative to IconContainer.
I did some testing with current and the frame child api gives access to parents of the multiboard Cell-Content. Which is enough for scaling & position.

container = BlzGetFrameByName("MultiboardListContainer", 0)
something = BlzFrameGetChild(container, 0)
row = BlzFrameGetChild(something, 0) --(0 to RowCount - 1)
cell = BlzFrameGetChild(row, 0) --(0 to ColCount -1)
IconContainer = BlzFrameGetChild(cell, 0)
TextContainer = BlzFrameGetChild(cell, 1)

BlzFrameSetPoint(spriteFrame, FRAMEPOINT_BOTTOMLEFT, IconContainer, FRAMEPOINT_BOTTOMLEFT, 0, 0)

I expect you know how to create the frame to display Frame Border Effect.
You need a short delay between creating the multiboard and accessing the content frames otherwise the game crashs.
No idea: about multiplayer, replay or observer sideeffects.

good luck

how to create the frame to display Frame Border Effect

Click to expand...

could you please let me know?
I succeeded in converting the example in this link to jass but I've also failed several times trying to make a sprite appear on the hero button for experimentation.