I've never been block by just pinging anything, but if that happens for someone, well... it'll simply won't work for that user. No big deal, that's the upside of being a free no-SLA service :)
As for my own side of the net, I asked someone at DO and I was told that it should not be a problem.
Every 5 minutes is more than fine for a free service (and do future upgrades for every 1 min or constant monitoring).
In any case let the user select their interval.
Nice work btw!