Sharing SPI bus with APA102 & n other peripherals ( faking an 'APA102 CS' pin )

adding a "small" ( 1 uF ? ) cap - how would it "smooth" the result ?

The ADC in the chip expects quite a low resistance to get accurate results (since otherwise the act of switching in the ADC itself would cause the voltage to drop) - so the capacitor gives you that low resistance :) It may not actually be an issue here at all - it's usually for 100k+, and you're not looking to get loads of accuracy (I guess even just 6 bits would do you pretty well).