NanoWatt,
This really is not a good idea. 300mw taped to a 20 foot pole is going to be really hard to be precise with, you will have beams going uncontrolled everywhere. At that power level you do have to be careful where that beam goes.
I am sorry to be a buzz kill but this has the potential to to go wrong and further fuel more stringent regulations. Lasers are not toys and should be treated with respect, . All that would have to happen is a stray beam across some aircraft or get the attention of some cop and you will get a lot more attention than you want. It won't be so cool then, in fact i would call that an epic FAIL.
Again the chances are slim that it would be a problem but...
chad
When the going gets weird, the weird turn pro.