I've made an attempt to write a plugin for Chrome that is intended to work the same way as the ones for Windows and Mac. So far it only supports serial ports.
Those of you who like to try it can get the source from https://github.com/fluffware/scratch_plugin.git
It works with my own extension and hardware and I believe it might work with the Pico-extension.
The Arduino-extension has at least one bug that prevents it from working.
The list of ports it tries to connect to is currently hardcoded.

I hacked together a chrome extension to replace the NPAPI plugin.

Since I see you are working on making things cross platform I thought you might like to check it out.

https://github.com/cactrot/scratch-device-host-chrome-app

My original post about it: https://scratch.mit.edu/discuss/topic/160398/


Also I see you found the same race condition I did.

It appears the khanning fellow hasn't looked at his project in a while since my pull request about that has been open for about 5 months.

https://github.com/khanning/scratch-arduino-extension/pulls