Just found this interesting discussion. It offered a definition:
This expression [for all you know] can be used in any context where the speaker wishes to convey that the listener is failing (deliberately or otherwise) to consider all the possibilities.
And some paraphrases:
it wouldn't be surprising if
it would make sense to think
You have no knowledge that would disprove that
As far as you're concerned, it is entirely possible that
You might very well assume that
All typical English style phrases that defy direct translation.
没准, 说不定, 谁知道 are all good, but the style is a bit off because the "you" part is omitted. So I propose this:
For all you know, she could be lying to you.
- for all I know -- 我也不敢肯定.
- for all we know -- 我们都说不准.
But sometimes it might be better to drop the pronoun and just use 说不准 and alike, depends on the situation and mood.