I'm of the opinion that the lenders couldn't do this but this isn't in anyway based on absolute fact.
Most lenders will lend up to 80% without LMI. Their reasoning is that there is little risk of property values falling this amount. Anything over and they are still not at risk as the Insurers pay them out. From the insurers point of veiw, they receive premiums (sometimes quite hefty) to cover any debts they cannot recover from a client should the need arise.
If the lenders were so concerned with falling prices then they have the opportunity to drop the LVR from 80% to say 70% should their risk management area see fit. As they have taken the risk they shouldn't have ant right to make a "margin call" on a property.
As other replies have mentioned, if you keep up your repayments the lender would have no cause to even consider this strategy (if it was possible). The way the bank make monet from you is by charging interest. They'd be mugs to try and have you reduce your debt as it would cost them revenue. It was also mentioned that the bank would need to individually value all their security on their book (well at least say in the say 90% LVR range) to find out it's exposure. This would be such a costly exercise.
They only other way of them finding out would be if you required additional finance and a valuation was done. In this case I feel your new application would be knocked back but the existing debt would remain.
The only thing I could see the lender doing is forclosing as soon as it was legal to do so if you were in arears on the loan. this would at least minimise their risk of further downturns.
To be absolutely certain of this I suggest you take your mortgage documents to your solicitor for clarification. I'd suggest that the clause (again if there is one) would be here and not in your letter of offer. I cannot ever remember seeing such a clause in a Letter of Offer and I've read quite a few in my time. I must admit I've never read front to cover of Mortgage Documents as there isn't a need for a MB as concerns in regards to Mortgaage docs need to be answered by the client's solicitor. And as a borrower (who should read evreything) I'm very slack and have never bothered ~~SMACK~~
Regards
Steve