No modulation at all. What I've heard so far, is everyone that uses chorus pedals on fretless, are using it to camouflage their shortcomings in intonation and relies on the chorus to mush it up, so you can well be off with your fingering. As suggested by someone else already, you can use a short slapback delay, and produce your own chorus by moving the fingers slightly forward and backward along the string to make the delay and your "vibrato" on the string sort of "chorusy". Compressors and EQ are ok though. Reverb should be used as little as possible. I think it just messes up the low end. If you play high up, in the baritone area, maybe you can use it a little bit.
Chorus pedals "directly" on the attack of the note seems to kille transient pick and definition. It muddies up the initial attack to with their 20 ms modulated delays. Me myself likes a long delay with modulation so the chorus is only heard in the background on the feedback of the delays.
Regarding compressors and sustainers, I think on a fretless bass, I still would love to (see other thread about sustainiac) have a pedal or effect to KILL THE DECAY of the string on a fretless. Especially when playing high up to mimick an upright properly. When playing high up on an upright, it's just a percussive instrument with a short thud, and no decay/sustain at all.