Set IP adress for the wifi hotspot on OSX
AFAICT, this is managed by the process com.apple.InternetSharing. When it is enabled, the file /etc/bootpd.plist is created. Here it has your IP address 10.0.2.1

However, this file is deleted and recreated when Internet Sharing restarts, so modifying it is pointless. I bet that the defaults command can be used to set a new IP address, however defaults read and sudo defaults read do not cite anything related to "com.apple.InternetSharing".

I know this isn't a complete answer, but if you find whatever is creating /etc/bootpd.plist you'll probably accomplish your goal. You may want to read this earlier question about Internet Sharing.