How about this for a warmup solution? Lets call it a Fast Warmup Device.
Take an EGR cooler off a modern engine (such as the Ford Focus diesel). This is a stainless steel one:
Attach it to an exhaust diverter valve Y piece:
That's an expensive one
You will be able to pick one up for free with a vacuum control valve from MKIV Supra turbo owners who've 'gone single' and will likely throw it away
Plumb the two together and feed a coolant pipe through the cooler, and there you have an exhaust heated FWD.
Unlike the above valve assembly the Supra one only has one butterfly in one of the two runners, so less than half the exhaust gas would go through the cooler, which wouldn't be a bad thing.
Supra EGCV:
Also a Y piece would have to be made up for the inlet from the stock exhaust, and chop & weld the outlet.... but that's not a problem for us professionals
The egr cooler (now a heater) would need to be sited a bit of a way from the exhaust system so that residual heat didn't get to it when it was not in use & cause overheating of the coolant.
A simple thermostatic switch could be used to switch the system back to standard exhaust routing when the coolant was up to temp.