The logo of postmarketOS was added to Nerd Fonts in v1.2.0, and it also shows up in the patched version of Meslo (installed via the package font-meslo-nerd from the Alpine Linux repos and ttf-meslo-nerd from the Arch Linux repos).

However, I haven't been able to find any references to postmarketOS in Powerlevel10k itself, so I'm guessing that for this to work, checking for the ID postmarketos in /etc/os-release needs to be implemented in Powerlevel10k as well.