I'm using the paid service from windscribe with a Pi as a VPN router for my lan. Works fine for me. (Pi3 acting as AP, DNS, DHCP, and router)
I can see no reason that the free service wouldn't work.
Unless they've changed things recently, you'll need to set things up manually as their limux tools have been built for a different CPU type - x86 rather than Arm.
Not sure what steps are needed on libreelec but essentialy you install and configure openVPN.
I do hope you're not intending to use the VPN for nefarious purposes (accessing licensed content without paying for it for example)
Attempts to contact me outside of thes forums will be ignored unless signed in triplicate, sent in, sent back, queried, lost, found, subjected to public enquiry, lost again, and finally buried in soft peat for three months and recycled as firelighters