Coolant going down - no idea of cause

Good question. Silicone is pretty heat-resistant stuff, up to maybe 200 or 220 ℃ in ordinary formulations, or as high as (I'm reading) 300 ℃ (572 ℉!) for special "heat resistant" silicones used for engine gaskets and such.

I think there are parts of the cat converter where temperatures can be higher than that, but I don't know if the distal ends of the exchanger hose nipples would ever get that hot. When the factory hoses are attached there, I don't think those hoses are even any fancy silicone. (But of course, when the hoses are attached, there's coolant there.)

I'm not sure much could go wrong from trying it to see, other than possibly heat-damaging the caps. What else were you going to use them for?