This seems like a huge oversight, but I can't find any reference in Goblet of Fire to Harry asking Sirius for help with this. Did I miss it? If not, why didn't he contact him?
Harry forgot to mention it.
After Harry sends off a letter for Sirius, he said that he could not have expected advice about the clue in the egg from him, because he was too busy telling him about Snape and Moody so he forgot to mention the clue in the egg.
“Harry scribbled the dates down on the back of Sirius’ letter, tied it back onto the brown owl’s leg, and watched it take flight again. What had he expected? Advice on how to survive underwater? He had been so intent on telling Sirius all about Snape and Moody, he had completely forgotten to mention the egg’s clue.”
- Harry Potter and the Goblet of Fire, Chapter 26 (The Second Task)
After he remembered, there was no chance for him to ask Sirius about the egg. Sirius’s reply to the letter Harry sent off there comes after the Second Task is already over, and is a very short one asking Harry to meet him and bring food.