As far as I know, TWRP installation should work. The point is that TWRP will not be able to access the data partition.
However, as I wanted to have XPrivacy, I came up with a workaround: To get XPosed, the only thing you need TWRP for is to flash the XPosed-zip file. So I did the following during the setup of my phone (see also the TWRP tutorial, which also includes the instructions for installing XPosed):
- Install TWRP
- Flash the xposed-vXX-sdkYY-arm.zip file via TWRP (for FP2 with Android 5.1 you’ll need the sdk22-arm version)
- Install the Xposed Installer apk
- Reboot
- Install XPrivacy apk (or any other module you’d like to have)
- Only then encrypt your phone!
I guess that steps 3 and 5 are still possible after encrypting the phone, but I’ve not tried yet.
Anyway, bottom line is: If you want to have the XPosed framework and its modules with an encrypted phone, backup your data, do a factory reset and then follow the above steps.
Hope, this helps. 
Edit: I haven’t been completely satisfied with the solution above, cause with each OS update I’d be forced to decide between letting go XPosed framework & keep my (encrypted) data or erasing all data to be able to flash XPosed again. However, I now realized that it should be possible to avoid that by putting the XPosed-zip file onto an external SD card, cause then it will be accessible by TWRP! I haven’t tried yet, but I think this should work.
Although that thought looks rather obvious and others might already have figured this out easily, I didn’t. And so I share it with you (hoping it helps somebody). 