I want to create a custom two-part epoxy resin sculpture of a robot, with Lego Technic inside as a skeleton. Will the resin get so hot it will melt the plastic?
Lego is ABS, which can have a range of melting points melts up to 200°C*. We should consider the lower end of the range just in case, which is often quoted as 105°C. Your resin shouldn't get that hot but you could test (a similar volume in a mould made of scrap, with one brick inside if you're really worried).
Generally, slower epoxies are cooler, and large volumes get hotter, so you can make it work by choosing a resin that works at low temperatures, and keeping the volume involved in any single cure down to a minimum. Depending on what you're aiming for this may mean multiple pours.
* Strictly speaking we shouldn't talk about melting points as it's amorphous, instead "glass transition temperatures"