Currently we don’t have a PWM driver, if you write one – please do let us know!
Note, the PWM header is not connected up for GPIO. If you look in Meta Flow SDK.Reference Manual.1-9.pdf on page 29, you will see the following:
The PWM/GPIO header is only connected to PWM pins on the SoC. The ‘GPIO’ next to the header is a silkscreen error. A number of other pins can be used for GPIO on a variety of other headers though. See the Pin Assignment section in the Minimorph Development Platform Reference Manual for a list of which pins support GPIO.
Have you had a look at the schematic (available from the Meta Insider download page)? You can find there the pins from the SoC that connect up to the LED you require.