T2 ICT, cooling period, and business trips

[zachstruck]

As an American whose Tier 2 ICT visa has expired but am still in the twelve-month cooling period, am I allowed to visited the United Kingdom for a short business trip with my US passport under the visa-waiver program?

[Frontier Mole]

The answer officially per the policy / guidance would be yes it is allowable.
Things to consider - business trips - under that visitor visa you are only allowed to do very restrictive activity. If you are doing anything other than the activity and it strays onto Tier 2 working type activity then things could get messy.
I would look at the business visitor visa conditions and make sure all the proposed activity is covered. I would also prepare a schedule of activity for your visit to provide a back up IF questioned.