Leo/Graves bot lane is infamously strong. Riven is not a tank, nor does she really siege too well, she's more all in; Mundo or Maokai would probably be better. There are also better tank junglers than Elise and Sejuani; Nautilus is incredibly tanky, for example. For mid laners; Ryze is gutted right now (pending rework), but he was a decent tankier mid laner, you might also be able to play like Mordekaiser or Galio or Vladmir.
I think it'll be better if you choose, rather than 5 tanky champions, pick just a sieging composition. The problem with just letting voidlings take turrets though is that they just get countered by waveclear.
The strategy could probably work, but I don't think it's optimal or really efficient.
EDIT: Just tested the item out in a few AI games real quick; the range on the minions is pretty short, and they aggro/target select in a very strange way, where if you put them in the jungle they might go to an unintended lane for whatever reason. You constantly have to move the spawner up if you want to contest objectives; I think the most useful is to get like, lvl 9 on Singed, go RoA and ZZ'Rot and boots and maybe upgraded trinket ward so you can ward jungle and stuff, put the spawner between turrets while you proxy, that way you can still protect the spawner (with your fling) and I think if you place the spawner correctly, you can take both turrets if you place the spawner well (haven't been able to test this for T1 and T2 turrets since the proxy farm took the T1 turret before I got the items, but it seemed like it kind of worked for T2/T3 turrets). I don't know if the item is core or really necessary on any champion, but it's pretty fun to use.