It's pretty common technique. People that do it on the regular sometimes carry big heavy-duty water bladders and just scoop up some water for ballast. You can only heel over so far using the boom. Some people hang weight from a halyard and that has the added benefit that if you pre-mark your halyard, you know you are heeled exactly right when the halyard is on that mark and the ballast is just at the waterline. We had to do this once and borrowed some 50-gallon barrels for the job.
Your math is a bit off though.
With a 60.3' mast height and a 5 degree heel, you only gain about 2 inches of clearance. To lose a foot, you need to heel about 10 degrees. But to gain 2 feet, you only need about 15 degrees. And 3 feet only requires 18 degrees. If I only needed 1 foot extra clearance, I would aim for a 20 degree heel because the cost of being off by a bit is quite high. To achieve 20 degrees, you would need to get that ballast about 20 feet outboard which is longer than your boom. Even to get that first foot of additional clearance, you need to get that ballast out about 11 feet, which puts your boom at 45 degrees. Assuming you can deflect your boom about 60 degrees, the maximum clearance you can gain will be somewhere around 2 feet. That might be enough for you. It puts you at about 12.5 degrees.
Or you could try letting some air out of the tires.
