Associate readyboost to background application

ReadyBoost's function is to cache certain writes that occur to the disk drive (paging files and system cache). Because reading and writing to disk is significantly slower than it is from reading and writing to RAM, Microsoft created ReadyBoost. It's purpose is to extend the disk-caching capabilities of your computer's main memory to a USB flash drive. It doesn't necessarily speed up your applications on the whole, just whenever the OS needs to write or read from the system cache.

When ReadyBoost is enabled, Windows 7 uses the USB device primarily for caching data that uses random input/output and small, sequential input/output rather than large, sequential input/output. This is due to the fact that memory on the USB flash drives are more efficient when it comes to random I/O and small, sequential input/output.

If background processes are running and the OS decides it needs to read or write from the system cache, then those processes will benefit from it, but there is no way to "tag" a program to use ReadyBoost. It's all up to the OS and what operation is being performed.