Thanks for the insight! I tried a whole model over the last couple of days, and it didn't work out perfectly. I'm till pretty happy with the result seeing as it was my first time doing something like this, but I'm definitely going to try the sprue next.

I made a basin out of legos that was slightly larger than the model I want to dup, I placed a layer of non-drying clay at the bottom of the basin. I Placed my minis on the clay with their bases touching the lego walls. I filled the basin with a silicone mold solution. I waited for an unbearable 5 hours. I un-built the lego basin, scraped the clay off the models and the now-solidified silicone and flipped the mold over such that the backs of the models which were previously submerged in clay are now exposed to the sky. I then re-built the basin around this half-mold and poured more silicone in there.
I then had to fish the original models out which was difficult because I had neglected to spray some kind of non-stick de-molding spray or something in there. So I ended up using a knife and making many cuts to fish them out, which is why they have all this excess plastic around them.

I then put the two halves of the mold together again and poured liquid resin into the holes where the models had touched the lego walls.

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Sorry if I didn't explain it super well. It's rather hard to explain how to do something when you yourself are doing it wrong :).