Looking up "catch 22" I came across this:
"A situation in which a desired outcome or solution is impossible to attain because of a set of inherently illogical rules or conditions:...blah blah" http://www.thefreedictionary.com/Catch-22
Yes, as you put it, it would be impossible due to the rules and conditions set by yourself since you are assuming the host receives your keyword along with a file size request.
But since no request is made other than a keyword, it would be impossible to know what file sizes the client is filtering on their end.
So the thought that a trojan would generate upon receiving a keyword request would be inherently illogical.
please correct me if Im wrong.