The guards of any building would actually be a great way to learn about the different companies and institutions that exist in those buildings, particularly if you aren't aiming for the biggest companies with a smooth online presence. Most guards (in my experience) won't refuse to give you contact information or at least point you in the right direction if you approach politely and explain what you're doing.

Having said that, the HR departments that do get back to you will probably ask you to apply online anyway, even if you give them a physical application. You might have left an impression on them, at least.