It never hurts to be friends with the security guards at the front desk, too. Maybe get to know the security guard(s) who are on duty when you arrive and leave work, and let them know that you understand that they are not responsible for keeping an eye on your bike. Ask them what is the best way for you to bring your bike in, and maybe they'll suggest a back entrance or something.

When I worked in New York, I used to carry my bike right past a huge sign that said "No Bikes or Rollerblades In Building" every day, and wave hello to the security guard who never said a word about it. He would even hold the door for me sometimes. I also used to bring him coffee and a bagel once a week when I would go back out for breakfast, and a slice of pizza if we worked late and ordered dinner.

Good luck, don't give up!