I found this excellent tool : [TOOL][WIN][v1.5]Take Screenshots in Recovery & Aroma - All devices in XDA devs forum.
You will need a Windows PC, an USB cable, and of course a working adb.
Tested successfully with TWRP, may work with any adb-enabled recovery.
The tool integrates all the necessary stuff (adb, ffmpeg, …).
At first run you will have to setup the tool according to your device screen, then it is ready to go
For some screens you may need to get the “bottom” framebuffer.
Also be aware that exiting the tool will exit your command-prompt session, and close adb.
May work with a pre-launched adb-over-tcp/ip connection, didn’t test that.