We are messing around with Custom Extensions in Reporting Services. Our extensions bring in a number of DLL's that we regularly update and remove as we play with our new toy. The problem is that invariably we delete one of the DLL's that isn't ours… bye bye Reporting Services. 
After a couple of attempts and a little searching I found out you can indeed set the private probe path (the path you see displayed as "Intial Private Path" if your looking at fusion logs) The CLR binder will then also check this path for DLL's. Thus allowing you to seperate your DLL's from reporting services'… or any other situation that may be more relevant to you.
Whack this into your app or web config. Also notice that it is relative to the application directory.
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<probing privatePath="bin\ECM"/>
</assemblyBinding>
</runtime>
Posted by adammills 