I want Optifine compatibility with the Direwolf20 pack because it can in theory increase my Frames Per Second (FPS). I am willing to accept no for an answer.
The FTB launcher allows you to point to external .zip file extensions to load when selecting your mod pack. You can select the Optifine .zip to be included as a "jar mod" when the modpack's own mods are loaded, and it saves you from editing the .jar yourself.
I've done this before, and it does install Optifine, and if changing visual settings, zoom, etc. is your goal it works just fine. Though personally I found my FPS to drop; I'm not sure why and uninstalled it before I did too much investigation.