My original answer stated that this would not work, but I have discovered that is incorrect. Poo piles that Larry Jr drops will in fact drop items. So far I have only observed hearts and soul hearts, but I would assume that the drop rate is the same as that of any other poo piles. The piles will drop items if the player pops them or if Larry pops them.
It may be slightly impractical to farm items this way because if you shoot at the poo piles you risk hitting Larry and possibly killing him, and you have to get hit infrequently enough to make up lost health with item drops.
So, it seems that you are correct that you could eventually fill the heart meter with soul hearts, but it would probably take a very, very long time.