From the documentation:

If required, HttpUrl and/or HttpHost can also be overridden in the web.config by adding the following app settings respectively: FullTextSearchHttpUrl and FullTextSearchHttpHost.

So I have set those values in my web.config. But is there anyway to set that on the fly in the code? (I want to base it on some parameter set on a node in Umbraco.

We have a different url for testing than will be in production. I have to deploy the site through dev->qa->prod and it's a painful process (i cannot change the process). So the prod version web.config has the official prod version of the url, but it's only using a temporary testing one now. So the site search is not working (and not passing the testing). And the client just changed that URL again, so now have to go through the whole deploy process to edit the web.config. (I know there is a package to allow editing of the web.config on production, but that really goes against all the source control deploy processing etc).

Also wondering how this affects having multiple sites with completely different URLs