Quote(just as long as they don't Dick Cheney me...)
Or worse, they could Dick Cheney's HUNTING partner you.... *bang*

Not much to add. I have to deal with some gov agencies but lucked out that my boss has a lot to lose should the wrong bit of publicity hit the papers. Gave me SOME negotiating power. However, I do not actually work for the .fedgov in any way so have nothing really to offer up.
I basically went to my HR person first. Described what was going to go on and left it at that for her to soak it in. Follow up meeting I provided a couple of books (thin, T work stuff) and some documentation. I was asked some questions and basically told her that I was passable and that I was starting to have issues when using the men's room (which she was surprised and our building is mostly open to the public). Stated what I expected and what I wanted from management. At this point the higher ups in the main HR department (large division) were brought in as was my boss and her boss who runs our entire division.
Next meeting was with the local HR person, one from the main division that we are a subset of at the moment, and my boss. The only other case they had to go on was a crossdresser who decided that he wanted to present as female at work (and uses the Men's room, etc, *from what they said*). They stated support for me and the expectation that anything that occurs would go away after a week or two. Strategy planning on outing myself followed.
Next meeting was me being told how I was going to deliver the message. E-mail was out since it was too easy to forward it to others outside of the company and to the media.
Next time around I presented the rough draft of the letter to be sent to everyone. Tweaking followed and I brought up the bathroom question again (remember, asking is polite demanding). They did not have an answer.
Meeting with my boss the next day, I presented what would be the finalised letter going out to everyone. Brought up the issue of bathrooms again and also "requested" a written statement from my boss, her boss, and HR stating that they support me in my transition. At this point, 2 business days away from T-Day.

Later on that day more wheeling and dealing ensues. In the end I dropped my demand for a written statement of support in exchange for use of one particular ladies room near my office and a paragraph written by me stating that they are aware of my change and that it will not impact my employment. Letter is now finalised and I start photocopying 150 of those things and stuffing them into envelopes.
Take the week off.
Return to work to shock and amaze everyone. Two things were continually said to me, 1) that I looked good and 2) that they were surprised that I did not change my usual routine at all. I still made my rounds and engaged others in conversation. Even though I was nervous (could not eat at all that day) I went in just like it was any other day out of the year.
Now I did schedule my transition just after my annual review (which was stellar, natch) so that I would have the grounds to raise a big stink if anything happened.
The far left and the far right at work did try to make things difficult but I refused to let them see me get upset, flustered, or anything. I was professional the entire time and brought up the issues with my supervisor during meetings that she had requested. Things were dealt with quietly.
Good luck and let us know how it goes.