I've put on a good deal of lean mass over the past year lifting heavy, primarily squats, deadlift & presses.
Would I be able to maintain my current muscle mass with just body weight exercises or a lot lighter weight? i.e. weighted pushups, etc? Or would I gradually lose mass over time?
Yes, you can maintain and build bulk with body weight exercise. There are a lot of ways to increase things like leverage and such to increase the resistance without weights. Also, non-gym based exercise (no equipment, no bars, no nothing) tends to build more on the core as well, since most exercises require core use.