i started a defret project, and it occured to me that while the bass wasn't coated with finish or had strings over it, i could maybe do something to it

are there any easy and safe ways to put designs on the neck? ie stencil w/ spray paint

No. think of the wear the fretboard gets from heavy wound bass strings. It's pretty harsh. An easy coating (stencil/airbrush) would not last long before looking worn and dirty unless you then epoxy coat it, which is a pretty time-consuming and skilled job to get a good even coating then sand and buff back.
Inlays are the way to go and they are fiddly and time-consuming too.

you COULD stencil/draw on the fretboard, then coat it wil polyurethane......

i did once.

yeah, hows a design finished with polyurathane stand?

You could stencil and then burn it in.