This video link shows how I did practice casting when I began DH Spey. We don't need a grass leader in Finland because we have plenty of water around, mostly still but not frozen when the grass grows but the winter is long and when March sun finally did warm the air my thoughts did change from fly tying to fishing and there also came an urge to do some pre preseason casting practice.
To achieve a Spey anchor in snow I did use a very long "leader" which I tied to something far ahead. A plastic bag filled with snow was enough. For my early years go to setup, a 15ft tip action rod and 35ft shooting head, I did use about 100ft of 55lbs Nylon mono. Just the same I did use as a shooting line.
The casting feel is not the same as in water but this type of anchor allows to learn complete casting sequences using up to 90 decrees of direction turn. Actually the forward cast feels pretty much the same it does in water when the anchor comes right and full power casts can be done so it excercises the whole body very well and can improve casting performance too. Line can also shoot and there comes nice line loop too but naturally the distance is less than in water because of drag and weight the long mono has.
Of course the mono eventually breaks but the cheapest possible can be used. If you use an Asphalt is should not be too abrasive after car tires have grind the stones smooth. Perhaps You can find other surfaces which can be used as well? I have not tried on lawn grass but it should be fine as long as long as the mono can slide sideways without too much drag and sticking.
If You began using this there are two things to concentrate: First the initial lift. There is no water drag and line lifts much easier to the back cast than it does in water but do the lift slowly. Slow lift is important for all fly line casting and for Spey it is very important and the importance rise when line head lengthens. Second You should watch how the anchor lands and not use too much back cast power. Slight slack is fine and line tighens immediately the forward cast is becan but you can expect to have good D-loops when you place yourself around there where straight mono joins the line tip. This anchor does not "blow" but if the back cast / sweep is too powerful there comes a wave to the line in D-loop which is an indication to ease up the back cast as it would blow the sustained anchor or wouldn't anchor the touch&go cast.
My Circle / Snap and Poke might look strange but thats how I did learn it when a short shooting head with a long leader is cast. I begin the sweep as soon as possible after the anchor set to keep the anchor light and when I pause before the forward cast and even Drift the anchor does not blow because there is no progressive high energy line loop anymore.