If I'm honest, my advice to the OP (hypothetical, as I am not currently FSA authorised) would be to not enter into any financial arrangements with anyone with a poor credit history under any circumstances.
I'm still not 100% clear on what the extent of OP's partner's adverse is; as you will be aware, mortgages don't go into 'default'. When he says mortgage defaults, does he mean missed payments/arrears? Possession proceedings? Actual repossession?
Either way, this is about as serious as it comes and shows a fairly high level of poor financial management. The lenders won't lend to her for a very good reason and I would encourage the OP to think very carefully about this.
Just as an aside, if, as an adviser, you did employ some cunning scheme to 'get around' this problem and it did all go belly up at some point, you would be in prime position for being sued for giving poor advice; it is fairly clear that it would not be in the OP's interests to enter into this agreement.
I really dislike this whole culture of trying to find a way around a certain problem as you put it. The 'problem' (that the partner cannot obtain credit) is there for a very good reason (that she demonstrably cannot be trusted with money).
I guess I see being unable to obtain credit as a result of a poor credit history as being in the same category as a driving ban, or a disqualification from holding a directorship. You have been given the privilege of being able to borrow money/drive/become a director, failed to manage that privilege in a responsible manner and therefore have had it taken away from you in order to protect others.
I just don't think it's a good thing to try and undermine this system.