Molten ABS is still quite a thick liquid, you'll have great trouble pouring it into any mould, and not have it solidify before it gets to the far corners. ABS starts to soften at 105C, but doesn't really melt until 230C or above (depending on the actual plastic) I've done bit of casting of white metal figures (which does melt to a very pourable consistency) and it's still a major pain. You _must_ heat the mould to above the melting point, and leave channels for the air to escape while the metal/plastic goes in. There's a bunch of videos/etc on the interwebs about casting metal. There's been a bit of discussion in the 3D printer community about using 3D printer extruders to push molten plastic into moulds. That might work if you could seal the extruder onto the entry of the mould. You'd still have to heat the mould, and I don't know what material would be best for it. I used plaster for metal casting (which is easy to heat) that that would be a nightmare getting out of the intricacies of a lego brick, and would probably only last a single brick.
Melting ABS does give off some nasty fumes, so be well ventilated if you try anything. You could also try PLA plastic, it's not what Lego uses, but it melts at much lower temperatures and is much safer and more pleasant to deal with (I've got a couple of 3D printers, and use PLA whenever possible, it smells like caramel as it melts :)
If you try anything, let us know!