The LibremKey does a few things and for neither it needs to be plugged in all the time while using a computer:
- stores your gpg private key (used to decrypt or sign e.g. emails or to decrypt your password store
pass) - generate one-time-passwords (otp) to login to services like https://source.puri.sm, https://sr.ht/, https://codeberg.org/ or many others
- store static passwords
- at time of writing only with the Librem notebooks: authenticate the start environment to detect tampering and decrypt your harddrive
- for experts: login using kerberos (ask @guido.gunther who wrote a very interesting article somewhere about it)
For all these use cases (and maybe even more I do not know) you can plug the stick in, use it, plug it out - keep using your computer.
Some people made some configuration to lock there screen when the key is unplugged, but that is not a standard usage and not configured by default.
You’ll like not find a ‘power injector’ for usb-c that’ll satisfy your needs - the power delivery (PD) protocol doesn’t work in favour of such an idea. The most simple power injector supporting PD you’ll find is the smallest usb-c hub with a power connector.
If this did change over the last year (I searched for a simple power injector, too) then please don’t forget to post it or even better put it on the list of tested accessories.