If there was nowhere to safe (implying a signpost is unsafe) I'd bring it inside.
-If there are objections, you could argue that the company pays hundreds if not thousands to maintain parking lots so people can bring their car and let it sit for 8 hours every day.
-It brings in dirt? Well, somebody is probably coming in and vacuuming/cleaning periodically anyways.
-If dirt an issue, perhaps you could volunteer to vacuum it up if you see it.
-In North America, that also represents ~50% or more of the square footage of the property. You could say that offering to bring your vehicle in with you and clean up after it is actually saving the company money.
-If there are more cyclists, band together and get a rack put in. In my experience, if people SEE that bikes are at work, its a visual indication that it can be done, and they are more willing to try it themselves.

I've never had to argue these things before, but you can quickly come up with a list of arguments in your favour.