A new version of TWRP for lilac is available fro download from https://sourceforge.net/projects/yoshino-testbuilds/files/lilac/
Details in this XDA Forum post
This version handles decryption of the data partition, so backup and restore of the data partition should work. I tried the backup and restore on a device runnning an unofficial build of LineageOS, encrypted with a pattern, and it seemed to work fine. One little quirk: when I press Restore, I see a message saying ’ Pattern is enabled. Pattern should be displayed before restore’. I ignored it and swiped to ‘Continue restore’. I hope the misleading message will be removed in a future version, but for now you can just ignore it.
The developer reported that decryption using a PIN also worked