It seems to me that the force from a chambered round kick comes from a whipping action. The knee is the fulcrum and the instep is the fastest moving part of the 'whip'.
Using the shin in this case would take away from the power of the kick.
That being said, there is a variation of the thai kick called half knee-half kick. The kick is thrown like a thai round kick, but the knee is kept bent (45ish). As the knee approaches the centerline, the kicker drives forward into the opponent. The contact is still with the shin but higher up, close to the knee. It looks like a chambered kick to the onlooker and is used mostly to punish the thighs or belly.
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