Looking at a long(ish) term fix shimming a 27.2mm post into a 30.9 st.

Frame is way outside of any warranty, doesn’t have a particularly extended ST above the TT and there would be at least the length of the shim (100mm) in the frame.

I know shims have been used for years so I’m guessing it should be okay but thought I best to ask the masses.

yep should be fine. The way seatposts and frames connect hasn’t changed so there is no reason for shims to not to be OK to use.

Until recently I would have said yes….

A few Hummer threads on here made me change my mind; at least with a Ti frame.

I wouldn’t have a worry about steel, though.

been using one for 2 years now – no probs. (alu frame)

Ali post and frame here and no problems.
i’m old enough to remember 27.0 posts shimmed into 27.2 frames with shims made from old coke cans…

I still use old coke cans to make a rear shock bushings.

Can’t you get a 31.6mm post and use an inverse shim?