I've used Liquid Glass or block sealer in older cars many times for cracked blocks n heads. Would it be possible to do on an inboard boat motor since they don't run radiator?

only if its a closed cooling system, does it use antifreeze or lakewater? if antifreeze then yes it would work, lakewater and it is just going to run into the lake.

no - i mean yall think theres a way to reroute or rig up in driveway to use it to fix cracked block or head without pulling the engine? could i plumb a radiator to one temporarilly to fix it or other?

It would have to be a pressurized system like a car has to force it out a crack.

If you close off the water intake and exhaust with motor filled with liquid glass it should`nt take very long to warm up and pressure up. I do`nt see it working.

if it uses lake water and not antifreeze you will not get it to work. the glass has to pressure up and stay that way for a little while, doing this with the intake/exhaust closed off will overheat the engine.

Highly doubt that you will get it to work. In my experiences with cracked blocks and cracked intakes, The crack is so large that even a welder would sit down and cry. If it was external like on the exhaust manifold, Sometimes grinding a V in the crack and applying JB Weld works.