I use max2play (LMS) with the Hifi Berry Amp2.
Is it possible to connect a chromecast audio dongle via an additional Hifiberry DAC+ DSP card using Toslink and select the input in LMS (wave input?)?
As far as I know that is not possible. Our drivers do not support the input from the DAC DSP. For this you would have to manually install the appropriate drivers and create a DSP profile. A simpler way with Max2Play would be the HiFiBerry DAC+ ADC. The analog input from the ADC is supported by us and can be used with the Wave Input plugin of the Squeezebox Server.