Well, I was able to use it as an effect. You put it in the audio track like a normal effect (choruse, delay, etc.). The audio should then be available at the preamp out. As an example: If you want to route the audio through the Arp's VCF then you could connect the preamp out to one of the VCF's audio inputs (ring mod, vco1, vco2,vco3, or noise gen).
How you say it,there will be no option to use an instance of the arp as an effect and at the same time control it over midi? So if i want to use it as an effect,i can't use my hardware controller to tweak the sounds?
I mean playing musical notes with a midi keyboard and playing the normal synth sounds. If you mean using a hardware controller to control sliders or knobs, I think you should still be able to do that.
I tried to use the Arp as a synth on a midi track and tried to route an audio track to it but Reaper crashed every time I tried it. I'll try again tomorrow as I just thought of a couple of things I might try (placing an audio sample in the midi track, some sequencers can do this).
Update: I tried again and was unable to route audio to the Arp when using it as a synth.