I am trying something similar with a mix of w clover, birdsfoot trefoil, black medic and plantain, plus a smaller amount of yarrow, sheeps parsley and burnet. (Don't know anything about some of these plants except that they are supposed to be low growing.) The clover is a small leafed variety, I have undersown it with spring oats early this month with the aim of drastically cutting inputs (the aim). I have inadvertantly had a white clover understorey which survived from a grass ley. It got a bit toppy but you can control without killing it using Allymax at a very reduced rate I am told but never got round to trying that because we killed it with Broadway star while I wasn't paying attention properly.