Not sure if I understood correctly what you are trying to describe.
You wish a singer to monitor himself without latency, but with software reverb? The idea seems quite good to me. You need a soundcard with direct monitoring and a mixer built in for this, and route the input to the headphones or whatever in hardware. The trick you are trying to achieve with the reverb is a good idea. I would try to only record the 100% dry signal, not the reverb you put in just for the singer to feel some space around his/her voice. Add a 100% wet (and leveled in to taste of the singer, but not passing any dry signal) reverb to the headphone channel. You probably need to mix direct monitoring and the software reverb somehow in the sound card, I'm not sure if every device is capable of this.
Also the reverb WILL have a slight delay, but if you keep it 100% wet so it passes no dry signal (which will come from the hardware mixer) it probably won't be noticed by the singer - and feel kind of like a slight pre-delay in the reverb, with maybe only the early reflections being a bit odd feeling if the reverb is advanced.
As for trying to monitor zero latency forget about any software mixers inside the daw. There is already delay by the audio buffers in it. You really need a sound card with a low latency hardware mixer inside to do this. Of course normally this is controlled with software, a mixer applet by the sound card manufacturer. Also it won't be total zero latency, but very low if the soundcard is of good quality & built for recording.
Another trick is to really tweak and tune a pc hard for realtime performance to be able to operate with sub 3-4ms buffer roundtrip latency dropout free for recording. Then the delay will be so small that it is almost unnoticable, even for singers. And you will be able to monitor with realtime effects at leisure, even affecting the "dry" signal. If they don't add a lot of latency, but they will probably use quite some more cpu than you are used to the lower the latency is tuned.
But I think renoise wasn't built for this. I'd use other software for recording. And I haven't yet managed to make it really stable at ultra low latencies, while other progs on my box can do the tight stuff after lots of tweaks applied.