Could a portable swamp cooler be be made to work better on high humidity days by using it concurrently with a room dehumidifier?

Yes, in theory, but impractical in execution. A swamp cooler normally pulls in outside air to treat, not inside air. If they are set up to recirculate inside air, they will over humidify the air, and cause sweating problems.

A dehumidifier will be adding heat from the compressor into the space while it does its' job, while you then have three motors running (dehumidifier, pump, fan). For that running price, I think you could air condition your space. Those energy use calculations can be run, and you could determine it for sure. Find an engineering student to help you; it sounds like an interesting set of questions.