I'm thinking of participating in my country's national brewers cup in 2020. Just read up on the rule document for WBC 2018 and I see that there's no restriction to the brewing device used as long as it doesn't use electricity.

Does this mean that its possible to do manual espresso methods such as Espresso, Portaspresso, Flair etc for participation in the competition?

It's a Brewed Coffee competition. Espresso is not "brewed coffee". No need to look for a loophole. Just do better than the next person.

Under "5.0 Standards and Definitions"

Compulsory Service 5.1 B "Competitors will be given eight (8) minutes of Setup Time and seven (7) minutes of Competition Time to prepare and serve three cups of coffee, each brewed individually, to three judges."

Open Service 5.2 B "Competitors will be given five (5) minutes of Setup Time and ten (10) minutes of Competition Time to present, prepare, and serve three cups of coffee, each brewed individually, to three sensory judges."

Seems pretty self explanatory.

5.4. D. Each of the three beverages should be between 120 and 375ml. If a beverage served is found to be less than 120 ml or more than 375 ml, that particular beverage (cup) will be disqualified and receive no score
Competitors are often disqualified for underpours. Will your espressos contain 120ml?
Aeropress is used ok along with popular diluted receipt. So if you do like that with manual espressos seems to be ok.