I'm not sure there is a single word you can substitute for "garbage" in this saying that will make sense in a data context (by the way, I am an information analyst). If you know that you have put "garbage in" then it doesn't really make sense to say in the same breath that you get anything other than garbage out. If you try to substitute it with a word that carries even some value, it doesn't really carry the full impact of what you need to say, which is that the output is wrong, and potentially misleading.
In my field of work we normally use expressions like "flawed" or "skewed" to describe aggregated figures which are wrong because of the process by which it was created - either bad data or flawed logic. If the end-user of a report is not aware of the logic behind the calculations or does not fully appreciate how the data going into a system may affect reporting then they may be looking at flawed data not knowing that it is incorrect. "Garbage in, garbage out" is the very phrase thought up to explain this, so I'm not sure why you want to change it.
If your intention is to state that end-users may get false data without realising it then my advice is to go with a word synonymous with misleading, because that is basically what it is if the data looks right, but isn't.
Garbage in, misleading data out.
Okay, so it isn't snappy - but I think it is what you mean to say. If you knew you were getting garbage out you'd put it straight in the bin and start again. The actual scenario you are trying to describe is one where the person getting figures out of a system doesn't know that it is garbage or something of little value.
Some other great words you might use instead include erroneous (wrong; incorrect) and fallacious (based on a mistaken belief), but the key thing is that your audience understands what you are saying and the gravity of it.
I do love some of the other suggestions you have had, but they don't really slot into the saying like you perhaps hoped. Sayings like "chocolate fireguard" are usually part of a simile, ie "as much use as a chocolate fireguard". But again, the reason I am shying away from sayings like that is that false data can be used! You can report garbage to your boss and he/she may never realise! The issue isn't really that it cannot be used, but that it is wrong, and that is what my answer is focusing on.