So, I emailed Speidel directly to ask them what size threads they use. They were nice but refused to tell me. The guy wished me luck and hoped I could find a way to add the TC but he still would not tell me the thread size. I don't get that at all. Why would they care if someone knows the thread size? There's more than one way to deal with proprietary threads - I chopped them off with a Dremel and inserted a TC bulkhead into the resulting hole. I even had space left over (just barely) to add a thermowell and a blow-off 90 onto the lid. It was tight but I am now able to add my hop bong on top.
ps - thanks
@Yesfan for the suggestion on the bulkhead.