Unfortunately Trulia deleted agent blogs, so I can no longer just link to it. But to answer your questions, the bankruptcy trustee needs to keep some funds to be able to do the sale. Otherwise the court will not allow it. There are two methods or retaining funds--a carve out, where the funds the trustee retains is included in the price, and a buyer premium where it is not (but is still disclosed to the creditor). The trustees who do not allow carve outs do so in the belief that banks are less likely to approve them. I have not found that to be the case, but you're not going to convince a trustee otherwise if they've made that conclusion. Buyer premiums do need to be disclosed on the HUD-1 form, and cannot be typically be financed, which is really the issue you are raising. They don't count as part of your down payment.
As to where the carve out comes from, it is just a deduction on the HUD-1, like the payment of another lien. It just shows the money going to the trustee rather than to a creditor. So if a buyer pays $200,000 for a property, $20,000 might go to costs of sale, $20,000 to the bankruptcy trustee and $160,000 to the creditor. An equivalent buyer premium transaction would be one for $180,000 with a $20,000 buyer premium, $20,000 going to costs of sale and $160,000 going to the creditor.
The bottom line is the trustee is unlikely to care that you want to make a larger down payment. They've already made the decision to only market to those who can come up with a down payment and their buyer premium, a decision which greatly reduces their potential market. So the difference to them of your being able to put 10% down or 20% down is going to be a difference they don't care about at all.