A Z80 system I built in 1999 had that feature. While I do not have a schematic, it's quite simple to acheive that. When designing the memory map one of the high-order address bits is used to select RAM or ROM chip. This is hooked up to one memory chip straight and via an inverter to the other. Using a DPDT switch to swap these connections, it is possible to boot either from RAM or ROM.

Using a monitor to program the RAM and then booting from it may be a bit tricky, though.
I had an on-board EPROM programmer to program the RAM on that Z80 system.

I hope this helps,

Regards,

Dincer Aydin