The Rannuncarpus is a very effective block detector. But you're GOING to need to use bore lenses if you want to use it as such (and stay pure botania). Your counter idea is likely more effective, because the Rannuncarpus only places above the detected block. Pistons will almost certainly mis-orient, and the redstone block + redstone mana spreader + bore lens trick only works there because the blocks beneath the livingrock and livingwood are obsidian. In your case you'd have to continually re-place and re-till the soil.
Unless you only do the wheat in rows (which does grow faster), at which point you could space them out a bit, and use a pair of sideways-facing redstone mana spreaders to place the block, and break both of them... but the redstone wiring required as compared to the slight added efficiency seems to be a waste to me. My vanilla water-harvest wheat farms worked just fine... actually I ended up lava-roasting chests full of seeds after every harvest (I build BIG farms in vanilla for some reason). Though you could get around the "gumming up" effect by having an additional spreader with a bore lens pointing "up" the row where the redstone will be placed, causing a periodic auto-clear. Heck you could use a diluted pool loop to make the "timer" for that one... if only the bore action didn't reduce the mana in the burst :-( Having to use a comparator to refill the pool just a little bit seems to be over-engineering when the hourglass works just as well.
If bore-lensed spreaders can break crops (which they may not be able to- I know they don't break flowers), the most efficient solution in my mind might be as follows:
1. Rannuncarpus places a redstone block on top of a fully matured wheat crop.
2. Redstone block triggers an adjacent bore-lensed spreader aimed downward and across the middle of the crop
at an angle.
3. Mana spreader fires a mana burst, which passes through the redstone block, the corner of the wheat crop, a mana detector, and into a block of obsidian.
3a. In doing so, the spreader breaks the redstone block and the wheat and triggers the detector. It does not touch any other blocks (save the obsidian).
4. Mana detector sends a signal to a dispenser and corporea funnel simultaneously, placing the seed stored in the dispenser on the farmland and requesting a new one.
If, instead, the mana burst cannot break crops, it may be possible to use a stationary piston to harvest the wheat, either by pushing the crop off of the farmland or, if that un-tills it, by moving the farmland out from under the crop. In either case, I'd still need a bore spreader to remove the redstone block. I agree with you in that trying to use a Rannuncarpus to place pistons would not probably not work very well.