I'm not sure how much ground you're interested in planting by this method, but I have a few ideas.
I guess It would be good to know: Are you trying to accomplish some soil-conditioning plantings as you are transitioning a area of ground into a planned use? OR Are you trying to arrive at the right tool, anticipating perennial use for the years to come?
Does your body weight force a spading shovel into the ground? Or do you have to 'kip' with your legs to force it in? Disc openers aren't concave like a shovel, but I think that the shovel would be a good indication of how much weight it takes for whichever (discs, etc.) implement to cut to the desired depth.
Low fi, one-time solution: Lay felled logs where you want to plant, wait for grass to die back, add a generous hump of compost or other arable material and just plant into that. As long as the grass is sufficiently dealt with I'm sure your plantings' roots will do the work to get where they nee to go.
An added benefit would be that you could move the logs just upland (if there is a slope) for a few days acting like a little check dam if there is water that could harm the plantings establishment.
Another idea is to make a wheel hoe 'seed drill' dealy-o like the ones listed above, with a circular saw retrofitted to be the disc opener.
I would recommend a lower profile blade with a scale saw kind of tooth pattern.
That 'saw blade' isn't low profile and I would be worried that it would cease up. The tooth pattern, I think would be ideal.
Maybe something like this blade:
If there is a lot of work to be done I would also recommend a BCS walking tractor. But I'm there biggest fan so take that advice with a pillar of salt - it's too much mechanization for some's taste. They do also produce a lawn edger, as well.
I hope this helps!
Andrew.
P.S. I think it goes with out saying that said saw is 'donated to the cause' (warranty voided, motor/bearings abused, mud slowly ruins electrical connections or plugs up exhaust, etc.)
