Thanks for the positive comments, gentlemen. I'm glad that the design at least has some redeeming features.

I got started with sculpting today - just the armature and some basic shapes. I'll post up some pictures the moment that there's something worth showing.

the hole in the front of the dress - not very AdMech, is it?
I fully agree with you and Marco that showing cleaveage, or even having breasts, is distinctly un-admech, and is probably virtually unheard of in the Orthodox Cult Mechanicus.
It was something that I had considered quite carefully during my musings on where the line was drawn on augmentation within the Sacred Barrier Cult. Including a mechanical part into the body, such as a bionic limb or lung, is strictly a no-no, as it contaminates the purity of the sacred machine spirit. Genetic modification and biological augmentation was something I also considered. However, a machine is really just a construct designed by man to fulfil a purpose or perform a task. Therefore, does it stop being a machine when it is made of meat rather than metal? Selective breeding is probably about as much of an augmentation process that a strict Sacred Barrier cult would allow. Selective breeding isn't creating something, but merely bringing about the conditions within which something might simply come into being. (Breed smart woman with smart man, and cross your fingers hoping they have smart babies...) So long as humans or other intelligences do not 'design' the thing, the cult has the belief that it is not a machine. Therefore, genetic augmentations are out, since these are designed, and are therefore a means of incorporating molecular machines into the human body. On the other hand, selective breeding is borderline, but permitted, since it doesn't involve design per-se.
In this light, I also started to think about self-mutilations, such as removal of extraneous body parts. However, this is really just another way of redesigning the body. More like chiselling away a piece of rock to make the desired product rather than adding things onto it.
The sacred barrier cult holds the belief that machines and humans are both sacred, but must be held separate. They tend to pray to both the Emperor and the Machine God, but not in the same way as the Orthodox Cult Mechanicus. When praying for themselves or other people, they would pray to the Emperor. When praying for the machines that they tend to, they would pray to the Machine God. This is due to the belief of the barrier separating humans from machines reaching not only into their physical existence, but also their spiritual existence. They dare not invoke the name of the machine god for themselves for fear of contaminating it. They only invoke his name to bless and protect their machines.
So, for instance, they might say: "In the name of the Emperor, I smite thee!" and "Machine God, oversee this lasgun and see that it fires straight and true." But never "Machine God, guide my aim."
I'm getting a little bit sidetracked, there... What I was trying to illustrate was that their belief system is just as human/Emperor centric as it is machine centric. To start cutting themselves up in the name of practicality would be an insult to their own humanity, little more than an attempt to be more like a machine by means of emulation. (Behaving like one.) To try to act like a machine is an insult to the Machine God, and might be seen as a condescending parody, which might incur his (or more technically correct - it's) wrath. Imagine how you might feel if you walked in on your servants, and they were doing impressions of you? Now imagine that you're a humourless git, like most Gods tend to be. Would such an impersonator go without punishment?
So that's my sketchy reasoning as to why the female members of the cult keep their mammaries, aside from the simple fact that I've already created a character who is a once-female androgynous cyber-castrati, and don't think that my shelf needs a second one.

Now, the reason why her boobs are on show is, I will admit, mostly aesthetic. The dress is mostly inspired by period clothing, where putting one's cleaveage on show was considered to be a normal way of showing off one's physique. I also thought that it was a good way to break up the otherwise plain, dull, red area of her chest. However, aside from the eye-pleasing nature of boobs in general, I felt that showing a little glimmer of vanity and putting herself on show might be just her own little way of expressing a bit of humanity.
Anyway, if I later change my mind and decide that her cleaveage shouldn't be on show, I can always fill it in with greenstuff to just make the "window" into an Admech cog... I'll probably make all sorts of adjustments as I go along with this model, so I won't rule out omitting the cleaveage.
"No, it's a she, dammit".
I already get this a bit with the Hive Maiden. Admittedly, she doesn't come off the shelf very often, but when players who are unfamiliar with her rather twisted back-story see her for the first time, it usually results in a bit of explaining as to why she's not just an effeminate dude combat servitor.