I highly suspect that 27.2mm being extended to 500mm is beyond the maximum flexural strength such a tubing would have.
there's a reason why many folding bikes come equipped with large diameter, long seatposts.
Thicker material compensates for that easily. I do think the real reason being they rarely need that length (folding bikes are about the only ones that needs long post and they have their own manufacturing for it).
I would worry more about the stress it can put on seat post clamp area of frame which actually bears brunt of torque.