Very few restaurants actually give out real butter anymore. 99% of the time it's margarine. Even McD's stopped carrying the whipped butter. Most of those packets are not tightly sealed. Like the McD's butter tubs, the film comes right off once it melts. Although KFC serves it in little packets like the ketchup comes in, that's the only place I know that does that. Again I don't think it's real butter.
I'd be more worried about it melting (not just being soft at room temperature), to me it never tastes the same once it's cooled again. In the belly of the plane, any temperature is possible, and really wouldn't trust it.
You could try like a quart size ziploc inside a gallon ziploc. Freeze the butter in the quart, then fill the gallon with ice around the quart bag. Or fill it with water and freeze it around the quart bag. That might keep the butter at it's best.
Of course anything happens, and there's going to be a great big mess.