Disclaimer: I'm not an attorney, and I'm not offering legal advice. This is only my opinion.
The wrinkle on Rocky's method of actually calling the lender holding the underlying note to tell them what he's doing is something I've never heard. I wouldn't dream of contacting the lender holding the underlying note, but it illustrates an interesting, higher level point. Most lenders, with a notable exception or two, don't seem to care what you do as long as they're insured and somebody keeps making the payments. They will never give their explicit permission for a Sub2 deal, so you shouldn't ask, but you can usually get the deal done anyway.
The variation of a Sub2 deal you described sounds like it would work, but I think a lender who really wanted to take action could come after the house in a foreclosure if they chose to take a hard look at the structure of your deal.
The Garn-St. Germain Depository Institutions Act of 1982 is what many Sub2 investors think protects them against the lender being able to call the note due and payable via the due-on-sale clause. It states that "... a lender may not exercise its option pursuant to a due-on-sale clause upon ... a transfer into an inter vivos trust in which the borrower is and remains a beneficiary and which does not relate to a transfer of rights of occupancy in the property.”
Many Sub2 deals don't include the Sub2 seller remaining as a beneficiary, and the seller is almost always expected to move out as part of the deal. Furthermore, having the right kind of trust is important for the protection needed in a Sub2 deal. Notice that Garn-St. Germain said that the trust should be an inter vivos trust. I think a lot of people use just any old trust which I suspect would leave them with little or no protection if a legal challenge were raised.
So... it seems to me that, if you're concerned with rock solid legal protection, you'd want to have a much more detailed agreement that keeps the seller as a minority beneficiary, describes how a rent/sale decision will be made for the property, and uses the right kind of trust.
I highly recommend that anybody reading this consult an experienced Real Estate attorney before pursuing a Sub2 deal. There's actually a lot more to it than has yet been mentioned here.
Thanks,
Robin