No there is no way. Memory used isn't by just one script. It is the accumulation of code that gets input when a single page is hit. For instance, when you hit the page to view a topic, it has to hit the database and pull all of that info, and put it into PHP arrays and variables etc. Essentially, all of the database info, and code is put into memory and compiled at runtime. There is no such setting that will make a script use less memory. In order to make a script use less memory you would have to change the code and what it does as a whole. If you're host won't bump up the memory limit to at least 16MB, its plain and simple that you have no choice but to change hosts.
And for the record, unlimited resources is the sign of an over-selling host. Nothing is unlimited. I can promise you, that if you started getting a HUGE site, that they would quickly force you to get a bigger plan or to move to a VPS or Dedicated server.