Is it possible to use ALTLVDS_RX processing single ended signals in Cyclone V GX?

Although I suspect it is a very bad idea, you could try the ALTDDIO_IN core. There is no reason to expect ALTLVDS_RX core to work with anything other than LVDS. I've used the ALTDDIO_OUT for DAC's that use single ended DDR signals and it works well.

You don't say what FPGA or how many SN65EPT23 chips you need, but your going to use a huge amount of extra board space and add alot of complexity to save a couple of FPGA pins. Does the SN65EPT23 have internal termination?