This question has been answered by BobRay. See the first response.
It's not a good idea to use eval(), which makes it difficult to do what you want. One method is to write the chunk's content to a file, then include it, then unlink() the file when you're done.The piece of PHP code needs to be available on an on-going basis. Having it in a snippet which is read and written to a file, included and then unlinked is probably more code than in snippet.
You can also just put the code in a snippet (created on-the-fly if necessary) and use $modx->runSnippet('SnippetName', $fields), where $fields is an associative array containing the snippet properties.
If you need to create the snippet code on the fly, you can use $snippet->setContent($snippetCode);.
If you do continue to use the file (and I don't really see why you wouldn't), I would put it under the core directory rather than the assets directory. I would also make it a class file and call the class methods in your other snippet. That way if it ever executes, it won't do anything.