The proper way of sealing it is with a gasket and sealant on the gasket, so it would be incredibly difficult to do properly without the pan clear out of the way for access to both surfaces for proper cleaning and installation.
It probably would be possible to drop the pan, cut and yank out the old gasket with cured sealant on it, then do what you could for cleaning, and just squirt sealant in a tube along the gasket surface, then putting the pan back in place and bolting it up, but it's a lot easier to type about it than to do it, since the space you'd be working in would be extremely cramped, and the time it would take to do it might exceed that of just disconnecting stuff and raising the engine to do it right.
Silicone on the exterior of a gasket seal isn't perfect, but it can be a stopgap measure for minor leaks. I've even used black silicone to seal leaking head gaskets on the outside of the head and block, and though it only works for awhile, it does work. For a seal that isn't leaking due to pressure, it would likely work great and last quite awhile, plus redoing it every so often isn't that big a deal. It's definitely a cheaper and easier solution, and something I'd at least try before yanking the engine just for an oil pan gasket leak.
