I thought about doing this a while ago, but I just don't think it would be as fun as gmods prop hunt, without physics
No, I'm pretty sure creating a datablock wouldn't create any lag. Basically, it's just turning a bunch of data into a reusable resource. The problems start when you try to create/load a datablock to the server and the clients connected don't have the resources to load them properly. Subsequently, those clients crash.
Dynamically creating and deleting datablocks wouldn't lag, it's just a bad idea.
If youre talking about static shapes props, this is what id do: If you have a ton of possible props a player could disguise as, I'd try creating a static shape and having it follow the player (I don't think static shapes can be mounted)
If the number of props is fairly small, I would create images using the shapes, hide all the players node, and mount the image to them.
If you use bricks as props, you couldn't have them smoothly follow the player, they would have to snap to the grid.