Note well, O noder: evidence rules differ quite a bit from place to place. The following is based on United States evidence rules at the federal level, proposed by the Supreme Court and approved by Congress. This limitation is suggested by the title, the beginning of the notorious "Miranda" warning which U.S. police must give when making an arrest.
Can my writeups be used against me in a Court of law?
The short answer is: almost certainly.
If a writeup were somehow relevant to a court proceeding you were involved in, it would "come in" as evidence. Relevant evidence is generally admissible. Fed. R. Evid. 401.
What about the Fifth Amendment? The Fifth Amendment provides that no person shall "be compelled in any criminal case to be a witness against himself". It does not apply to physical evidence (you can be compelled to give a blood or urine sample) and more significantly for our present purposes, the privilege against self-incrimination does not apply to documents. A writeup is a document, not live testimony.
Privileged professional communications? Communications made to professionals (your lawyer, tax preparer, doctor, psychotherapist, priests) are "privileged", that is, the professional cannot be forced to repeat what you have told him or her. Obviously, a writeup posted on E2 may be read by doctors or lawyers or priests, but was not intended just for that professional. The presence of anyone other than the professional waives the privilege. And professional privilege applies only to statements made for the purpose of facilitating professional services: you can't make facts "privileged" by telling them to your lawyer.
So what do you say, when the prosecution offers up that daylog you wrote about killing your roommate, and you have wisely rejected your public defender in favor of a pro se defense (perhaps in an effort to prove to the court that you are insane)? You state boldly:
OBJECTION! HEARSAY! May I voir dire the witness?
Don't ask me how to pronounce "voir dire". I personally like to pronounce it several different ways in the same proceeding, just to show what a down-home folksy guy I am. Would you trust someone who knows how to pronounce obscure Norman French legal terms? I wouldn't.
What does voir dire mean? Literally it means, "Judge, if you let me ask this bozo a few questions, I will show you something." In terms of U.S. legal practice, it means two things. First, it refers to interviewing prospective jurors to show bias. Second, it means you get to be a butt-in-ski and disrupt the flow of the prosecution's case to prove the point of your objection.
The prosecution must have a summoned a witness to provide a "foundation" for the writeup as evidence. You can't just come to court with documents and waive them around. Documents don't, as some lawyers like to say, "speak for themselves". You have to have witnesses to explain the documents. This would be especially true of an E2 writeup. A witness would have to explain what E2 was. A witness would be necessary to prove that l33t haXor, the author of the daylog which described hacking a person to bits, was really you. If they don't have a witness, the hearsay objection is sustained and you win, since as we all know, if the glove does not fit, you must acquit.
Assuming they do have a witness, then, you ask that witness...
Isn't it true that many writeups on E2 are fiction? Isn't it true that many writeups on E2 are posted by "trolls" and other fictional characters?
At this point you will have either (a) convinced the Court to exclude the writeup as unreliable, or (b) convinced the Court that you are not competent to stand trial, or (c) well on your way to the lethal injection platform to receive your recompense for hacking your roommate to bits, since this writeup is about U.S. law, and here, capital punishment is still allowed.
Disclaimer: IANYL (I am not your lawyer).