letterboxing 1 pixel on top...

[Selur]

and what about this:

This is a simple filter that fills the borders of a clip, without changing the clip's dimensions.

Use it if you want.
Hybrid has custom sections which allow adding custom scripts additions.
According to the README.md it seems simple enough to use.

core.std.LoadPlugin(path="PATH TO libfillborders.dll")
clip = core.fb.FillBorders(clip=clip, left=0, right=0, top=0, bottom=0, mode="repeat", interlaced=0)

You could even integrate it into the GUI, like mentioned here.

Cu Selur