I'm not a lawyer either - but you'll probably run afoul of several laws / ordinances.
As noted above, there's probably laws related to putting statements (of advertisement) on others' buildings. Think about it this way.. would you think it'd be perfectly okay if Domino's set up a mini-projector and projected their ad on your place?
Then there's probably laws regarding advertising in general - you'd have to check your national/state laws and muni/city ordinances. I.e. your city might have regulations for the placement and size of statements of advertisement in general.
On the non-advertising side... you mentioned a 50mW laser. That'll probably put you well into needing a permit (variance) for public laser display. Google on those terms and you'll find the info you need on that one, including the FDA document for the U.S.
That said.. yes - you can always just try and see what happens.
Best case scenario: nothing (other than positive comments).
Semi-worst case scenario: cops stop by, check with the office, find that you have no permit, and get you fined or worse.
Worst case scenario: somebody decides to take a stroll through the beams and looks into the laser (or, at least, will claim such).. find that you have no permit for the 'show', and sue the pants off of you.
On the other hand, getting permits for the various aspects isn't likely to be cheap or speedy process either.
Personally, I wouldn't risk it. But if you really want proper answers.. you'd have to talk to a lawyer.
