Ok, figured I'd do an update on my hire, in case anyone else can benefit. For reference, the job is a salaried position with the US forest service.
I accepted my job offer, and then immediately emailed HR and told them I'm undergoing a gender transition (haha, supriiiiiise!). They in turn got in contact with our regional civil rights director.
The civil rights director discreetly informed my new supervisor/upper management and then called me to ask a few questions about my transition timeline, making sure they get my preferred name on things, etc. She was also super excited, because apparently management at my local office is scheduled for a training session about gender identity and transgender issues in a couple of weeks. This had nothing to do with me, it just happened to be on the schedule.
My new supervisor apparently took the news pretty well (although I bet he was pretty surprised lol, since he met me awhile back 'en femme'). I'll be working with only a handful of people, so he'll be making sure my work environment is accepting. They kept stressing over and over how they want to make sure my privacy is respected, and that I feel comfortable at work.
Phew...heart rate finally dropping below 90 bpm for the first time in 48 hours. Overall good news, but this stuff is stressful.