Meerkat,
I understand where you are coming from. It's tough to put a 5% deposit down if you are not 100% sure you can secure the 95% LTV mortgage.
Your best bet here is to apply for the MIP for the property/asking price you have in mind - prior to signing your Provisional S&P and hope that the bank can approve your mortgage application.
The banks are inclined to take your application less seriously without a signed Prov S&P but push them a little bit and they should process your mortgage application.
This beats the possibility of signing the S&P, only to find that you can't complete it and losing your deposit!
The other option is to buy a property where you know you can afford the 30% down payment.
Banks like Standard Chartered and Hang Seng also offer an 85% LTV for investors that does not need approval from the HKMC. The approval process is not as strict and this could be another good option.