In LineageOS 16.0 this works: long-pressing any accentable character on the hardware keyboard (e.g. 'a') brings up a selector box allowing you to pick any accented variant ('à ', 'á', 'â', 'ä', ...) using the touchscreen. The box looks a bit rustic, like using a design from previous Android versions, but it works. Not sure about the later Lineage versions 17.1 and 18.1.
Note that this is for the original Pro1 only. We still know nothing about software for the new Pro1-X. The LineageOS ports for the Pro1 will no longer work because of the different SoC. A new port will be necessary and prob