Well when i have used it in the past, its usually what you will do, "i'll cross my fingers for you" but yes you can use it that way to show that your wishing someone luck

Americans don't need to use "have" in this instance: "I crossed my fingers for you" can mean it was done a week ago or a second ago. We can also say "I have my fingers crossed for you", if you must use "have". "I'll cross my fingers for you" should be for a wish that will happen (we hope!) in the future.

I would normally say 'I have crossed my fingers for you' - if it is current. Or past, would be 'I crossed my fingers for you'.