I'm not a expert on this topic. My first language is not English. So don't make fun off me when I try to post something.
The way I understand it: There is two type of mortgages. Prime + X% and/or HIBOR.
Prime + X% is the most expensive in terms of the interest you have to pay. So the best option is to go for HIBOR. (This option is still protected by the Prime + X% but with X being a bit higher).
Here's to scenarios for the CX scheme with 2 different banks.
1. BOC. You have a HIBOR mortgage, term 20 years and the interest rates go's up. Therefore you simply increase the 20 years to lets say 25 years to be able to pay it with the CX allowance.
2. HSBC. You have HIBOR mortgage, term 20 years and the interest rates go's up. But you cannot increase your term if interest rates go's up so the extra amount now have to come from your own pocket. (OUCH!)
So to be safe you go HSBC Prime +X% or BOC HIBOR.
Cheapest BOC. Winner BOC.
This is how I understand it. I might be wrong so check this with your bank.
Use Frances, with her contacts at HKMC you will get approved.