I've been thinking more about this, Darshani. How about, instead of putting which you would prefer in your birth plan, you put something like:
In the event that the baby is malpositioned (ie posterior) please encourage me to change position frequently during labor; to labor on my hands and knees, to perform pelvic rocks in between contractions, to freqently get up to empty my bladder, to use the birth ball, the tub, and other positions to help encourage the baby to turn. If the nurse or doctor have any suggestions for positioning, I welcome all their help and encouragement. During pushing, please remind me to fequently change positions, from side lying to hands and knees, to squatting. Please suggest any and all positions to help the baby turn and help open up my pelvis. I know that some babies remain posterior no matter what, so I welcome and encourage all your help and expertise in my healthy, normal vaginal birth of this baby, whether she decides to come out anterior or sunny side up.
This way, not only are you telling them that you know that your baby might be posterior, but that you know that you might have to work harder. They know this, too, but they don't know how motivated you are unless you communicate it to them. Penny Simkin always talks about wording your birth plan in the positive. When you word it like the example above, you are envisioning yourself doing all these things to help open up your pelvis and get the baby in a good position. When you word it properly, you get your caregivers to envision themselves helping you birth this baby, taking an active, positive role. Also, you are showing them that you have thought this out. You are an educated, strong mama, and you know your options. They are going to recognized this. Finally, you aren't overtly wording it in a way that says "I know you just like to do c-sections because it is the easy thing to do, you lazy bastards." Rather, you are engaging them, asking for them to use their experience and knowledge to help you have a healthy, natural birth. It doesn't hurt to stroke their ego a bit, in the form of "Oh, I know you have so much experience, I know you know lots of tricks to help me have a nice, healthy, vaginal birth." A carefully worded birthplan strokes their ego a bit, while also clearly (though not defensively) conveying your expectations and empowerment in a positive fashion.
Good luck! Think positive thoughts (and remember, a posterior baby isn't necessarily a bad thing! It is the position the baby chose, probably for a good reason. Posterior DOES NOT EQUAL C-SECTION OR VACUUM OR FORCEPS! Where I work, we do not do any of those things, yet we do not have a problem with moms having to transport for posterior babies. It just isn't an issue.). I'm sending lots of empowering and positive thoughts your way!