On reflection, I think a 6mm hole is probably crazy.
I kinda think a small 3mm hole on the back side of the post would hold up? It’s a basic, oem alloy post. Probably the best type to try it with. It’s thick n soft. The hole could be touched with a countersink drill bit to round the edge (at least on the outside of the post).
More details required, I think, I'm having difficulty visualising what you're trying to do. Di2 cable down the TT, into the seatpost, and up through the top? How will you remove the seatpost? Or thread the Di2 wire through?
Through the TT yes, but it will need to exit.
On reflection, I think a 6mm hole is probably crazy.
I kinda think a small 3mm hole on the back side of the post would hold up? It’s a basic, oem alloy post. Probably the best type to try it with. It’s thick n soft. The hole could be touched with a countersink drill bit to round the edge (at least on the outside of the post).