The SH3707 is MBX based, so not a Dreamcast compatible SoC. Its creation was commissioned for potential use in portables -- power consumption was suitably low, as such -- as well as amusement (arcade, pachinko, pachislot) devices, where it saw its primary implementation with the Aurora platform.
Though similar to the SH3707 which was commissioned specifically by Sammy/SEGA, Renesas's SH-Navi SoCs, like the SH7770, were actually the chips which were used in car navigation devices.
SEGA did commission the design of a real Dreamcast-on-a-Chip back then which was, as mentioned, used by Pace in set-top boxes.
What I find most intriguing about the SH3707 is that it's an MBX platform with, according to Simon, support for hardware translucency sorting.