Well, it turns out that my assumption that the car failed from overheating was wrong. I dug into the inverter, and discovered that two of the IGBTs had died. The only thing that could really cause this, I reckon, is voltage spikes caused by switching. The problem was that when I removed all the DC bus capacitance (except for some small snubbers near the IGBTs) I didn't count on all the inductance I had added to the system in the form of cabling and the battery pack itself (essentially a big loop of wire). When the IGBTs try to turn off, this inductance causes the bus voltage to rise until something happens, in this case catastrophic failure of the IGBT itself. Pictures of the carnage soon!
Luckily, I was able to source off-the-shelf replacements, which will be arriving along with some bigger snubber caps later this week.