Maybe creating function similar set_transient() to write into file?

[wikicms]

For example:

set_transient_to_file();

Most have nothing to add, all described in the question.

[scribu]

The whole idea of transients was to have a caching mechanism that's guaranteed to be (somewhat) persistent.

The most straightforward way to do that is by falling back to the wp_options table when there's no other caching backend, such as memcache, APC or yes, even a file-based caching backend.

Therefore, set_transient_to_file() makes no sense.