Well you have had the Zodiac Spear method so I thought you may as well have the Zodiac Escutcheon too and this is a LOT easier than the Zodiac Spear because the item percentage is 60% which means a lot more patterns will work and you don't have to wait for a 5 hit combo though that will still work.
At the end of this post will be a vid you go to where you can see me getting a Zodiac Escutcheon from the Barheim Passage. It may look quite complicated as to how I got it but I had to do certains things in order to achieve the right location in a specific pattern. There are many of these patterns and I will list them all here but first I'll explain yuour basic set up.
I had one character (Vaan) with a Ribbon and another (Balthier) with the diamond armlet. I have to say that this Zodiac Escutcheon chest is a pig to spawn as early RNG patterns just do not sem to like it. You are going to have to hope that it spawns fast when you want it to. I tried it entering the zone normally, hitting myself once, twice and three times. It is still tough to spawn. In the vid I hit myself 3 times before entering and it produces a combo. The chest was there so hopefully if you get a combo when hitting yourself before entry that it will be there for you too.
Ok, on entry I cast immobilise on Balthier and then made my way to the Zodiac Escutcheon chest using the Immobilise Glitch to clear all enemies along the way. When you get to the dead end passage where the chest is, first check to see if the chest is there. There are 2 chests in one spot. Walk up to it and if it flashes 'Treasure' quickly then you know both are there. The ZE chest will be the first one you open. Use the glitch to clear remaining enemies but don't go all the way back to the chest or the glitched enemies respawn. Put the immobilised character back in the reserves, go back to the screen and then switch characters. You are now ready to begin hitting yourself. But what are you looking for? Study the following patterns and watch for them. Because the RNG can move a little they may not be exact but if you get a (say) 2 hit combo on the 27th hit then it is likely the 2 @ 26 pattern so work from that. If you get (say) a 3 hit after 21 hits then it is likely the 3 @ 23 pattern because there is no 3 hit combo at 21 hits. All subsequent combo mostly stick to the pattern with the odd exception. So here is what you are looking for and what you must do when you get certain patterns. Please remember to turn your gambits off during the final combo. If your character hits themself again you will lose your shield.
a) 3 hit combo after 23 hits - Continue - 4 hit combo after 29 more hits - Continue - 5 hit combo after 28 more hits - Open chest and get Zodiac Escutcheon
b) 4 hit combo after 56 hits - Continue - 5 hit combo after 28 more hits - Open chest and get Zodiac Escutcheon.
c) 5 hit combo after 82-86 hits - Open chest and get Zodiac Escutcheon.
d) 2 hit combo after 26 hits - Cast Curaga - 4 hit combo after 27 more hits - Continue - 5 hit combo after 28 more hits - Open chest and get Zodiac Escutcheon.
e) 3 hit combo after 33-36 hits - Cast Curaga TWICE - 5 hit combo after 48 more hits - Open chest and get Zodiac Escutcheon.
f) 2 hit combo after 21 hits - Cast Curaga - 3 hit combo after 41 more hits - Cast Curaga - 4 hit combo after 7 hits - Continue - 2 hit combo after 2 more hits - Open chest and get Zodiac Escutcheon.
g) 2 hit combo after 18 hits - Cast Curaga - 4 hit combo after 56 hits - Continue - 5 hit combo after 28 hits - Open chest and get Zodiac Escutcheon.
h) 2 hit combo after 18 hits - Cast Curaga - 3 hit combo after 21-23 hits - Continue - 4 hit combo after 29 more hits - Continue - 5 hit combo after 28 more hits - Open chest and get Zodiac Escutcheon.
i) 2 hit combo after 18-20 hits - Cast Curaga - 3 hit combo after 21-23 hits - Continue - 2 hit combo after 37 more hits - Cast Curaga - 2 hit combo after 9 more hits - Open chest and get Zodiac Escutcheon.
j) 4 hit combo after 4 hits - Continue - 2 hit combo after 10 more hits - Cast Curaga - 3 hit combo after 41 more hits - Cast Curaga - 4 hit combo after 7 more hits - Continue - 2 hit combo after 2 more hits - Open chest and get Zodiac Escutcheon.
k) 3 hit combo after 56-63 hits (approx) - Cast Curaga - 4 hit combo after 7 more hits - Continue - 2 hit combo after 2 more hits - Open chest and get Zodiac Escutcheon.
If you come across any different patterns please let me know because I may have missed them or they could be one of the patterns here but a little further along the RNG pattern. Remember, if you don't see an exact pattern then look for one that is very close and take the appropriate action. If I find any other info I will let you know as soon as I get it.
Good luck!
Zodiac Escutcheon Vid:
http://www.youtube.com/watch?v=ihV3oILLSU8