Bidirectional SPI switch?

You can't route a bidirectional signal without an explicite direction information.

I asked about the SPI master and slave roles, because I can hardly imagine a SPI network, where the peers change their role spontaneously. At least, it hasn't anything to do with known SPI standards.

You have similar problems however with true bidirectional lines, e.g. an I2C bus or simply the data lines of a processor bus. If a FPGA is intended to route a signal of this kind, it must be supplied with an additional direction information, or it has to decode the communication protocol and determine the actual data direction from it's protocol knowledge, if possible at all.