I've never done it or heard of anyone doing it.Most times I don't think its necessary unless its a 2 sided part.Its not a horrible idea,if what you are casting is to be a 2 part mold anyway.Idally though...I think if you attempt to cast a part w/the sprue attached that the pouring hole will be too small and trap in too much air.I think the casting may be riddled with voids and bubbles.You also need a second hole to allow for air to be forced out when pouring the resin in.Unless you have a vaccum chamber to de-gas the resin and remove bubbles for solid castings.I'm not saying it won't work,but it be an expensive waste of silicone if yu trying to cast a complicated part.
I can tell you how they are molded, and what is needed to make a tree of parts. It can be done without injection molding, provided you time the cure of the resin correctly . The important part is molding the parts in two stages, and having channels cut from the sprues to allow expansion, and contraction, as well as diminishing bubbles. Im have been able to successfully mold 30 parts on one cast with only minimal errors and about a 90% success rate on these. I had to recast a blown mold and it actually came out better than expected, with no expensive equipment, You will always have errors on a small percentage of parts, there is no other way around it unless you have expensive equipment...... but Im not a millionaire. If you want some tips...pm me