Re: has anyone tried this?
Get a sheet of corrugated card/metal/plastic, and do a test spray on that first. Once you can spray without runs, I would progress to the car. Remember loads of light coats (applied whilst the last is just getting tacky) is better than one coat thats too thick with runs.
Minor runs may be removable with cutting compound (slightly beefed up t-cut) which is a very fine abrasive which whilst wearing the paint down, still leaves a nice unscratched finish.
Also make sure you get the proper plastic primer, cos paint on plastic occasionally reacts. Also the bumpers can flex a little, and normal primer comes apart from the plastic, leading to flaky paint.
If you don't mind additional expense, forget that stuff, and go to your local paint shop and ask them for colour matched bumper coat. This is paint that is a bit more flexible than normal. I've seen this used surprisingly effectively on leather seats by a mate of mine, so it must be a bit more flexible than normal paint which would just flake off.