You can now evaluate expressions in the execution trace! Go to any transaction in the "Transactions" in the left-hand navigation, then go to the "Debugger" tab, and click on the "Evaluate" button:
The Eval feature is currently only supported for Solidity contracts, and we're working on supporting Eval for Vyper contracts as well. Choose any expression you want to evaluate, paste it into the box and hit the Evaluate button:
This works for both simple and complex expressions:
You can read more about other Debugger features and Advanced Trace Search here and here.