Well, I spoke with my HR lady today. Turns out the problem with my hair had nothing to do with length - it was just a matter of tidiness. One day last weekend, I walked into the daily standup meeting with an admittedly messy ponytail - it was like 80 degrees in the security office with no fan! Apparently, someone complained about how disheveled it made me look. I even know who, because I distinctly remember getting a very unpleasant look from one particular employee. Not that I really care, she and I have never butted heads, she's just a very strictly professional sort when she's at work. But I digress, I'm rambling. Bad habit.
Moving forward. My HR lady told me there was no need to cut it, and that off-the-record, it was a really good look for me. I wasn't really violating any company policy, just wound up toeing the line. As long as I keep it neat, and eventually start pulling it back once there's enough to uniformly pull back with nothing extra hanging down, I can wear it as long as I like. I never even had to bring up being transgender.
Needless to say, I'm tickled pink and breathing easy!