This is similar to the topic discussed here.
Generally, we say zachin l'adam shelo b'fanav, that you may acquire something for someone else if it is for his own benefit (even without his knowledge), except when your acquisition is at a third party's expense. However, in the case of acquiring a lost object on another's behalf, the gemara (Bava Metzia 10b) rules that it may be done even though it is at the expense of all other potential finders of the object. Ramban and Tosafos differ, though, on the reason for the gemara's ruling. Tosafos say that since you have the ability to acquire the object for yourself, you may do so for your friend as well, even at the expense of another. Ramban says that since that third party in the case of the lost object is not any one specific person with particular rights to the object, he cannot prevent you from acqiring it for your friend.
Thus, when it comes to the parking space, Shimon is trying to acquire the space on Reuven's behalf and at the expense of Levi. According to Ramban, it seems this would definitely be okay, because Levi does not have any personal rights to the parking spot, and this is essentially the same case as the lost object. According to Tosafos, it's not as obvious. Can Shimon potentially acquire the space for himself without an actual car? Perhaps you can say that since Shimon can pay for the parking space himself and store his belongings there if he likes, so too he may save the spot for Reuven as well.
If, though, there is a law prohibiting Shimon from standing in the parking spot, then first we would employ dina d'malchusa if it is an enforced civil law, and second, Shimon may no longer have a halachic basis according to Tosafos' opinion, since he cannot acquire the spot for himself without a car and without breaking the law.
Of course, for practical rulings, CYLOR.
