Yes, I've done a century and many near-centuries (75 and up miles) on my hybrid. Personally, and your mileage may vary as they say, I find the upright posture on a hybrid or mt bike easier on my lower back problems and have more pain on a road bike. That being said, I can have sudden attacks of lower back pain just sitting in my office chair at work, so go figure, it's not something I am going to let stop me ... I guess that's why God invented Naproxen and Ibuprofen ....
OTOH, you will definitely take longer to do a century on a hybrid or mt bike - in my case I am not going for speed, but distance when I ride my hybrid, so I don't care - when I go on long rides I do it when I have "all day" free, usually on a Saturday, so it's no pressure and I make a lot of stops - everything from stopping to take photos to shopping to having a meal to stopping at my office to drop excess gear or clothes or even a couple of time to feed the fish when I forgot on Friday and catch up with the weekend security guy (a fellow cyclist so I've gotten to know him).
Triathlon - dude, only if you seriously don't care where you place ... Or you're such an elite runner or swimmer that you'll make up the poor showing on the ride in the swim or run. Otherwise, forget it ... I mean, you can, but you're at a minimum 2 mph disadvantage from the starting line.