How to pull in dynamic content in hook_filter_FILTER_process?
Depending on your settings, hook_filter_FILTER_process() will be invoked (albeit in a round-about way) from within the drupal_get_form() and node_load() functions.

So at the moment you're simply creating an infinite loop that PHP cannot escape from (node_load() calls hook_filter_FILTER_process(), which then calls node_load() again), hence the out of memory error.