It's not really true to say it's an honour system - you are obliged to either have a ticket prior to boarding or purchase one on board. Of course, if no ticket inspectors come on board (at least 99% likely, see below) then you will get off scot free. If the machines are playing up then you would likely be able to blag your way out of it by putting on your best 'stupid lost tourist' impression to an inspector whose English probably won't stretch beyond 'hello'.
Having said that, despite having lived for 12 years in WAW, I don't ever recall seeing a ticket inspector on an airport bus - and for a night bus the chances are infinitesimally small - so you could just play the odds. I think one reason is that the ticket inspectors can barely speak a single word of English, and the airport buses are likely to have lots of people who don't speak any Polish. They can find easier pickings elsewhere.
Also, you don't board from the back. You board from any of the doors - front, middle or back.