You mean with a hypothetical future hardware device, as similar as possible to the Librem Key, that doesn’t currently exist? Sure. However …
I believe that this is not possible i.e. the data lines are used in the negotiation that happens in order to implement USB-C PD. However with back-to-back power negotiations, you can get the effect of “only the power lines connected” - so even if you connect to an untrusted USB-C power source, you are not exposing data lines all the way through from the Librem 5 to the power source.
(If the Librem Key is your primary source of trust on the Librem 5 then it probably isn’t ideal to expose the Librem Key directly to an untrusted power source.)
Of course a completely different (future hardware) approach is a second USB-C port on the Librem 5 - so you can charge the Librem 5 using the existing USB-C port even when a USB client device is connected to the hypothetical second, say, data-only USB-C port. (My assumption would be that the hypothetical second USB-C port would not be on the bottom of the phone.)