I've been keeping an eye on this post, and see you have no responses. I went searching for an answer to this predicament you're running into with your opponent posting their arm.
From what I gather, most 10p students are going to have picked up on the flow from invisible collar to muddy waters. In MTS 134 coach Eddie reviews a huge amount of material on the rubber guard-area you have been working on-I recommend this, as I've watched it several times but I don't want to lead you in the wrong direction as I'm learning as well.
I will say this: when the arm is posted, muddy waters isn't going to work. In this instance once you've done the double bag, go into kung fu then into sorcerer (I believe it is called). Here you have to be very flexible, Coach Eddie even says you should be able to do a lotus without the use of your hands.
Again, watch the MTS episode, it will give you tons to work with and hopefully clear up your rubber guard game when your partner posts.
I'm going to start working on this myself, as I dig what Coach Eddie was going over and he threw in all sorts of different paths.
MTS 134
01:22:48 Invisible Collar
01:47:57 Invisible Collar to Muddy Waters Option