The people were kind of safe in the domes; it's just that there was barely any food. So how could Belthasar help them? He didn't seem to have much more food than them either.
Going with the "He crossed the labs and visited the domes" theory, we'd have to assume he'd be able to hold his own in battles, so he could have very well gotten the seed for Doan way before Crono&co get there. The fact that he didn't makes me think he was never in those domes. He stayed wherever he ended up after the OPI and built his dome right there. Which makes me think the only way the others could know there was a crackpot in that dome, or even that there was a dome, would be some form of radio or phone or telecommunicator or whatever. It would only take one short conversation to confirm, yes, he's there and yes, he's nuts.
Belthasar-"Hello, out there? I found this device, I believe it's for communication."
Young Doan-"Yes,
oh god, yes. There's someone here! There are
people here! Tell me, who are you?"
B-"Oh, I'm Belthasar! Thank you for asking! Have you heard of me?"
D-"What? No! Are you hurt, sir?"
B-"No, no, I'm just fine. Here with my Nu. How are you doing on your end?"
D-"Dude, we're fucking
starving! Do you have any food?!"
B-"Nope. Just my Nu. Have you ever met a Nu?"
D-"This guy's fuckin crazy. How do I turn this phone off? Yeah, fine, just pull out the cable, we do
not want to talk to
this guy again!"