I live in an 100 year old house in Michigan. I have suspicions that the flashing around my chimney is leaking as the plaster in the ceiling directly below the chimney & roof point is getting soft and the paint is peeling. I intend on doing it the right way and replacing the flashing and/or ensure the seal is in good shape. The problem is that its winter and I cannot safely get up onto the roof to do that. Are there products that I could apply from the attic to help block the water from coming in until I can fix this the proper way?
No. That would be like sealing the bottom side of a sponge--moisture will travel through it to emerge in other areas.
You can probably accomplish the repair even in winter. A water hose will quickly melt snow and ice from the areas you need to access, or use scaffolding and other hardware to work over it.