Agony,
I've been thinking about your question for quite a while now.
I suppose the first consideration for you is cost (money and time) vs. benefit. Only you can answer that one.
I think a lot can be done. My big problem here is that I can't figure out how thick the 'shell' of a fiberglass mannequin is. So I cant visualize how best to make cuts, minimizing damage.
Below is a link and some photos for a site showing a guys work in Mannequin Makeovers. He's accomplished a hell of a lot with just the painting....I'm truly amazed.
Skin
Smooth-On.com might be able to tell you about how you could get silicone or latex or something to adhere to a fiberglass skin (I'm assuming that all the modern mannequins are now fiberglass)
Eyes
I don't see why you couldn't put acrylic eyes into the holes you carefully cut within the eyelid edges. Then paint eyeliner and install fake lashes top and bottom.
Boobs
It's conceivable that you could use the existing chest as a 'positive' for a mold. Make a boob mold. Pour Silicone boobs. Then cut out the old boobs and install the silicone ones. Tricky...but conceivable.
Easier...buy some Danni Ash boobs or some others you like (Goldman's???) and install them.
Bum/Vagina
Similar ideas to boobs, but this is a critical structural area. Use fleshlight or one of the popular Japanes "boeing" inserts?? or just figure out how to install a premade pussy/ass combo???
Moveable Joints
You'd want to start with a mannequin that had a pose that didn't distort shoulders/hips/knees/other too much. That way she'd look more natural in various poses (some mannequins, for example, have the shoulders thrown way forward in a sort of "attitude" pose...you'd want to avoid that)
I'd think that, if you studied Japanese Ball Jointed Dolls, you would find inspiration into how something rigid (well, rigid parts) is transformed into a very poseable piece. As well, study the Japanese love dolls that are not contiguous silicone pieces. You don't have to engineer their types of joints...I bet you could do a lot with bungees.
I can envision something like this: you make an angle cut in the hip, separating the top of the thigh from the torso. You use fiberglass (or similar) to round off the top of the thigh...now the thigh top sort of fits in the hole in the lower torso. You draw it up from the inside with a bungee (cut her head off to get inside...see below)...etc.
Poseable head
Carefully cut the head off at the top of the neck. make a rounded removeable cap for the neck top...or do something else...the idea is....to figure out how to get the head to sit "perched" on top of the neck so that it wil go left right and up down. you can cut a hole in the top of her head (a wig will always cover that) to get in to set up your rig that holds the head on....again, maybe a bungee.
Take a look at this guy's pages. Very informative!
Be sure to look at his "Before/After" section
http://www.mannequinmakeovers.com/