Brazing the plug in and the crack would work,but I know it would be impossible to get a torch in there and not burn everything withing a foot of it up,and also risk the cracked area spreading..the block would need to be near cherry red hot to let the braze rod flow in properly and getting it 100% clean would be tough too..
I have seen guys use lead on engine casting flaws or rusted pits,and on body panels,it is an art to get lead to bond to cast iron or steel though--it doesn't require a much heat as brazing does,so the risks are lower...a guy I know that had a 62 Corvette with a 283 had the block crack when it froze ,without enough antifreeze ,and he had the block "stiched" at the crack at a machine shop--they drill numerous holes so close they overlap like an "8" and then tap them,and put brass machine screws in the holes--then "solder" everything with the lead..
It worked great too..
I know how it sucks to have a potential "fatal" flaw in an engine,my pickup's oil pan keeps my heart in my throat knowing it now has 2 bolts "stripped" in rot holes and RTV slathered over it to keep the oil in,and that could let loose suddenly and destroy the engine before I could even pull over and shut it down on a highway..(but at least the pan could be replaced,not by me though!)..
I've had other engines with cracked blocks I puttied up with PC-11 Marine Epoxy and coated with POR-15 that never leaked,but I was always expecting a failure at the furthest spot away from home..trusting epoxy isn't a good feeling at all when you know metal should be there instead..
Hopefully that silicone plug mashed under the plug will withstand the 60+ psi of oil pressure on a cold start and not soften enough to fail after the engine gets up to operating temperature..I bet it'll work for longer than you think..