You can try VaM, the free version lets you play scenes. If you search by the virt-a-mate tag, you will find VR HuCow simulator, which is a quite decent weight gain scene.
I, personally have the creator key and have been learning how to use VaM, so MAYBE, in the far future i will upload something myself. Although i already have a couple of projects planned so i don’t want to promise anything just yet.