I'll take a stab at this.
You have to remember that the universe is an intelligent entity you're dealing with, that's what makes I/M possible. As intelligent, it has a will of its own. In order to get it to manifest things on your schedule rather than its, you'd have to somehow figure out a way to get it to play ball.
I think it's possible, but it would take a far greater understanding of the way our universe works than what the LoA affords. Understanding the role non-material entities such as spirit guides and angels plays would be one key. The other key would be the ability to manage your own growth.
The reason LoA is "allowed" to work is because using it facilitates growth. Now, we typically don't have the inner resources required to completely manage our own growth. Sometimes that new car you want isn't completely in line with what your Higher Self feels is your optimal growth path. This means that car gets delayed for the express purpose of your growth. This means you're getting taught a specific lesson by the delay.
The Universe and your Higher Self are not averse to making you wait years for your manifestations. If you want to cut that time down, you'd need to facilitate communication with these entities that are providing you with the things you want, and you'd have to actually listen to them.
Which is easier said than done, because we always feel that we know what's best for us. Which unfortunately means that unless you're willing to become teacher's pet in the Universe's classroom, you're stuck with waiting for your rewards when it sees fit.