This is the kind of thing that's terribly useful, but simple enough to easily build on stripboard. I like the 'bonus features' though, they might make it worthwhile for me to buy a couple if I ever get around to casing my amps nicely.
A couple comments. The Omron relays are pretty expensive. I think the Panasonic TQ2 series is a direct substitution, and seems to cost about half as much. No reason not to use the cheaper part here, IMO. Also, is a $2.50 18A MOSFET really required to drive the relay? I'm thinking something like the $0.25 2N7000 (200mA) or $0.35 BS170 (500mA) should be sufficient. If a tiny little 2VA transformer is used, it simply can't provide enough current to warrant the IRLZ24N as far as I can see, and besides, the relay coil current shouldn't be more than 100mA. Maybe you have another reason for using this particular part? Spending 1/4 of the total BOM cost on an grossly overengineered FET seems a bit much to me.
Edit: It'd also be nice to have a pads for +12V/GND. Could be useful for other 'standby' electronics, and I think there's room.