Loadcontrol with vitual path leaves DLL file open

I've built a simple plugin architecture for a .net 4.0 web app where developers embed ASCX controls in DLLs, then upload the DLLs into the bin directory of the hosting application which then calls their controls through the mechanism described here.  This is working fine except when it comes time to upload a newer version of a custom control DLL; it seems that the mechanism by which LoadControl and the virtual path provider fetch the control from the DLL leaves the DLL file itself in an open state for some reason.  When someone tries to copy a DLL of the same name into /bin, the write fails because the file is open.  The workaround is to go into IIS and restart the web application before uploading new files, but I'd rather not have to go through this step.
Is there any way to clear the lock on the DLL file after a control has been ...