I have a number of functions which, before doing any real work, need to do the exact same pre-processing routine of the passed parameters. As such, they all start like this...

local p1, p2, error = preprocess(param1, param2)
if (error) return error --early return

Is there a way to use metatables to inject this kind of default behavior into functions, without having to copy/paste that boilerplate code each time?

not metatable but varargs can help:

function preprocess(fn,...)
 local args={...}
 — check args
 —
 — call real function
 return fn(...)
end

function do_a(a,b)
 return preprocess(function()
   return a/b
 end,a,b)
end

function do_b(a,b)
 return a/b
end

— wrapping style 1
do_a(12,35)

— wrapping style 2
preprocess(do_b, 12,35)

Thanks @freds72
Funny enough, I do something similar in another part of my code, but for some reason couldn't recognize the pattern being applicable in this case.