The MHTML is an interpretation of the RDL language upon exporting a report definition. I am not sure if many of the advanced options of SSRS you would be available to do with exporting (drill downs, collapsing, etc). You can collapse with Excel though, I do know that.
Could you not develop an HTML page that calls to the server or else give the user direct access? Different formats will do different things but the direct report will always have more power than an 'export' format of it. Before going too far down the road of doing this I would ask: "Why do you need to do this?" Is a certain user having to have it in a detached format from SSRS?
There are two things I can tell you you can do depending on your level of knowledge of code and what you want to accomplish:
You may embed the report in existing HTML as a property. It is similar to:
< Form id="frmRender" action="http://(servername)/(webservicelocaleofreport) method="post" target="self">
You can code in a language of either C# or VB.NET a report viewer that can call to the webservice to VIEW the report live via SOAP web calls.