I have no idea about that; why don't you write them and ask them?
Again, how would I know the answer to that? You haven't even described what software your "chat forum" is using, provided any information about it's configuration, or produced an error message related to it.
From looking at the error message you provided, it is not even clear than iconv is in any way involved; the error message is reporting that your script can't the function bcdiv() (included in libbcmath) -- one of the BCMath Arbitrary Precision Mathematics Functions.
According to that manual page, and a phpinfo() run on one of my domains, that does not appear to be enabled on DreamHost's PHP. From the manual:
"These functions are only available if PHP was configured with --enable-bcmath."
If support was actually talking about this, instead of iconv, I agree that it is not installed at DreamHost "by default" - but they are two different things.
You can recompile your own PHP if you want, and enable it in that way.