The best I can think of is 僕が知る限り + fact + とは限らない. Does this work, are there better alternatives?
If I understand you correctly, the phrase you're looking for is more about entertaining extreme/absurd possibilities, or humorously saying how little you know about the matter, rather than a sincere “to the extent of my knowledge”.
Variations on “知る限り” will work when you are being sincere, but it doesn't really work in a sarcastic way. I'd say it's closer to a “to my knowledge” or an “as far as I know”:
As far as I know, he's a good person.
When you're just putting out some extreme possibilities, you might consider variations on “可能性もある”. Literally, it means “there is even the possibility that…”, but I think the sentiment can be close:
35 years is such a long time, you could become unable to work, or for all you know you could be dead.
- あの人はあまりにもよくテレビに出ていて、毎回 別人の可能性すらある
He's on TV so frequently, for all I know it could be a different person each time.
Profiles can be completely fake. For all you know, the person may be lying to deceive you, trying to involve you in a crime.
(Examples taken and edited from the internet.)