You can legally do this, but it will not be easy. You cannot distribute a binary in this way, if you do your patch becomes GPL so the first person who buys your patch can legally give it to everyone else. That means you have to sell the source code to your patch and your customers have to build the binary themselves. You need to ensure that your customers understand that they cannot distribute any binaries they make. (they probably can distribute it within their company but they need to have some process to ensure that their employees know they cannot distribute this)
If anyone is thinking about this, make sure all sales include your lawyer: you will need to ensure that your customer understands all the subtleties otherwise the courts might decide your patch is GPL.
