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Yes, the Interrail site says it is ok. But I have experience of some bastard at Gare du Nord making up his own rules so I place a premium on first hand observation!
"For Eurostar trains, the following information applies:
Adults are permitted two items and one piece of hand luggage free of charge, and children are permitted one item and one piece of hand luggage.Sports equipment: Items under 85cm at their longest would count as part of your luggage allowance. Items over this length would need to travel as registered baggage, excluding skis and snowboards, which can be counted towards your luggage allowance.
Bike Bags: A bike bag with a maximum length greater than 85cm would need to travel as registered baggage. Bags under this length can travel as part of your luggage allowance."
I would be of the opinion that if you get it under 85, it goes through smoothly.
If it gets into the scan i don't think it is worth their time to stop you boarding that way.