Do you have, and load, the file `/etc/bash_complete` or an equivalent directory? It defines a bunch of completions and extension facilities beyond what's built into bash. If you have access to them, you can probably just use
complete -o filenames -F _command trickle
It will complete the first argument of trickle as a command, and will then try to apply appropriate completion rules for subsequent arguments. But it depends on the shell function `_command`, which is defined in the above file (in my Debian system, at least). YMMV on other Linux distributions, and the file doesn't seem to be present in Darwin (OS X 10.8).