I'm legally separated, how do I hold my husband liable for loan payments that he dafaulted on but agreed to pay?

You have a separation agreement which is a legally binding contract between two individuals so the principals of enforcement are the same.  You would sue your husband to enforce the agreement.

That being said the issue of what is dis-chargeable in Bankruptcy that arises out of Family Law is very complex and difficult to discuss fully here.  It appears that the type of Bankruptcy will matter: chapter 13 versus Chapter 7, how the debt is categorized as spousal support obligations can not be extinguished in bankruptcy since the law changes in 2005. So how your separation agreement is written will matter. 

There may also be ancillary issues here like notice that should have been given to you is the loan agreement has your name on it.  Remember though that the business loan lender is not a party to your separation agreement and not bound by its terms.

You need to seek good legal advice from an attorney in your area that is familiar with bankruptcy and family law.  Ask if your legal bills would be recoverable from your husband as well.  It may be in your agreement or under the law because you have had to go back to court to enforce it.  Good luck.