Depends on state law and the language of the contract. Probably need a local lawyer to know for sure. It wouldn't cost that much to have one read the contract and opine.
But since you'd have to be a compleate idiot to pledge any collateral for something like that, it's not like they can just come repossess your car or house. If they actually go to court and get a judgement that would probably be taken out of current wages... IF they know where you are and who you work for.
Also practically, is the contract amount worth the legal fees and hassle for the employer? Usually not, typically those who try to enforce it are not trying to get their money back per se, but rather to make an example to encourage other pilots to serve their time. So a small company might be less inclined to pursue the matter than a larger one with a lot of pilots. Also right now, most pilots are sitting tight anyway so no real need to make examples.