Per https://community.platformio.org/t/request-to-add-platformio-support-for-earle-philhowers-arduino-pico-raspberry-pi-pico-sdk/22285.

As simliar to e.g. STM32 and AVR with board_build.core = .., it should be supported to switch to a different Arduino core implementation, the most popular of which is https://github.com/earlephilhower/arduino-pico.

Needs a PlatformIO builder script, a new framwork package and the necessary platform scripts changes here.