I have the big boob problem as well... recently i have gotten a bit obsessed with corsets, because they provide so much support without any extra help.
So, i got the idea to work corset boning into some future cosplays...
your costume looks like it has a shirt underneath the armor, i would suggest turning it into a corset of sorts, put ribbing at the top but not the bottom so it would not show up, and use a stiff material or coutil for the material.
rigilene boning looks like the best idea, it is not too extreme but can easily be worked into your costume to provide extra support. Also, this looks the easiest to work with and can be cut into whatever size you need, and you can also order plastic tipping to go with it. I imagine there might even be a way to work it into the bottom half of the costume so that it is not so noticeable.
Its details say that it can be sewn into garments by hand or machine, so I imagine it would not be too noticeable like other boning would be.
I ordered it recently, so has not come in yet, if you think this might be a good idea I will make a note to report back to you how it works.
OH also that site sells underwire, i would suggest buying that as well, and turning that undershirt into a cage of support!
.... PS having a huge bust for this cosplay will work very very well, I hope to see pictures when you are done =3
Edit: forgot a few things.
the armor, if you are going to have it support your toy toys, needs to be supported so it can do so. so think about how you will get in it, how it will attach to your costume, and how you can give it more support.
Also, use double sided boob tape on the cups to keep the toys in the toy box.
Another tip, make a paper machet (sp) mold of your torso while wearing whatever support you are going to be using under the costume for a base or at least a model for your armor. =3
this costume is meant for big boobs, so it should look amazing when you are done, post pictures I want to see! .. in a totally none pervy way of course.