I'll begin this next post with apologies, as I know everyone's flashed 6.1 ages ago, and here I come reporting "bugs".
Tried running the 4.3 camera just now, and was immediately greeted with SystemUI stopping. The same camera ran fine on B03. I've renamed both the included camera and gallery (.bak), and have cleared dalvik and cache for prosperity's sake. The camera works fine, I should note, SystemUI just insists on closing.
I'll attach a logcat below, although I understand it's probably a little too late to be bringing these things up. And that's fine.
I did exactly the same. And I am running it successfully. I can tell you the steps that worked for me :
1. Delete/Rename/Move Camera.apk and GalleryGoogle.apk from /system/app.
2. Add 4.3 camera apk to /system/app & change permissions to 644 root/root.
3. Clear Camera data from App Management.
4. Clear cache/davlik.
1. Remove Camera.apk & GalleryGoogle.apk from B6.1 zip.
2. Add the 4.3 Camera App to B6.1 zip.
3. Flash the zip like a normal B6.1 app.
P.S. I normally add/remove few apks to zip before flashing e.g. GoBackup, MiFileExplorer, AdFree etc.