Sec. 527
http://www.bankersonline.com/regs/201/sscra.html#527 tells us the loan must be to the SM or the SM and the SM's spouse to qualify for the 6% rate. But I have to say, I wouldn't want to fight it. The decision is yours as the law is specific and your SM isn't obligated, but do they commingle all debts and income, was their budget affected by the SM joining?
As to foreclosure, Sect. 533 is similar in that it is discussing the obligation of the SM and you indicate the SM is NOT obligated to pay.
Sec. 533. MORTGAGES AND TRUST DEEDS.
(a) MORTGAGE AS SECURITY-
This section applies only to an obligation on real or personal property owned by a servicemember that--
(1) originated before the period of the servicemember's military service and for which the servicemember is still obligated; and
(2) is secured by a mortgage, trust deed, or other security in the nature of a mortgage.
I believe the lender has leverage, but also has reputational risk regardless.