To start, these loans will need RESPA disclosures if they're to consumers, if these are commercial requests they wouldn't.
1. If your borrowers do not get their construction completed before the end of the interest only period. Modify or refinance the loan.
2. If the construction phase has over-runs. You can either refinance w/ new money. Or you could give them another loan secured by the 2nd lien (if enough equity is availsble) or another form of collateral.
These loans will also be reportable to HMDA as purchases.
This should help, I know I'm just skimming the surface though.
Can't is not an option.