Hi Mark
Respectfully I suggest you need to re-frame the question - how do I create a deal that works?
Is it possible one of the quick turn models may provide guidance, 6 months sounds like the length of an extended close to me. Surely you can find a fellow flat lander

who is more experienced with those methodologies.
If you have access to good 'hard money', look to arrange bridging for the vendor to get their new place, the sale is to you or your investor with assignment on closing, permission to rent during the bridge/assignment period, get the purchase agreement on the other side, and be prepared to place a short term second if the 'rent to own' client isn't actually able to qualify for the whole mortgage on closing - no need to build in escalations and worry about appraisal relative to marked up value, allow for some fluctuation in the market, consider some contingencies, and get it heavily papered.
Charge a consulting, finders or advisor's fee, whatever terminology is acceptable for all concerned, collect on the monthly basis in addition to the rent (no equity build) if necessary, with an additional lump sum at closing. If you have to take the second build in some incentives to payout quickly, or conversely build more profit into the second.
This all begs two questions; why they are going to be able qualify in six months but not now - does this factor actually still work against them then and given the relatievely short time horizon what is the rush? Without answers to those questions you can't assess your risk (at least what you are attempting to transfer). Will you really be able to extract sufficient money out of it to warrant the work in the first place?
Let me know if you pull something off
best regards Brad