The guru on my area uses a program called "Hoeflins" (developed by an old pro trainer Gus Hoeflin). Lots of arm and core exercises - no weights.
Some posters sons have used it and maybe can chime in.
When his pros train during the off season its 3 days a week Hoeflins, while simultaneously doing their throwing programs (long toss and mound). My son did it for many years and never missed a beat in his throwing program.