I'm planning an extended trip to 220V country and have all dual voltage electronics. I'd like to bring an extension cord and current tap (light socket with additional outlets). Are these designed for specific voltages or is the voltage concern only in the appliance itself? I cannot find one that states 220V but if they just carry electricity maybe it doesn't matter...

Each electrical device, no matter an appliance, cord, or power strip, will be labeled with the voltage it is designed for. If it does not specifically state 220 volts, and only states 115-125 volts for exqample, that is all it is rated for, and using it on a voltage it is not rated for is risky.

Power cords with an IEC connector are rated at 10Amps/250v. If you check a wholesaler such as farnell Search Results

They supply the details.