Is there a way to /whois an entire channel?
Kinda like /who #chan, but I'd also like to see more detaild whois information about the users, like which other chans they're at.

I don't think it is possible to do /whois #chan.

You could write/download a script that uses a loop to whois the whole channel, or the selected users. I'm sure there are many scripts available that do this. Note that whois scanning some channels is likely to get you kicked or klined.

-genius_at_work

you could try /who #, I know this is not the same. But it will give you all the info you need in one command.