If you send a "singlefield 1" message to jit.grab you can throw away one fieldset, but that cuts your vertical rez in half.
There's a simple de-interlacer shader included with MAX:
(Cycling 74 folder / jitter-shaders / convolution / cf.deinterlace.jxs)
Or you can try vade's de-interlace shader v001.deinterlace.jxs (attached to this post - seems to be gone from the internet these days?)
I think both shaders do simple field mixing, so they look better than "singlefield 1", but you won't get full sharpness either. You can read about smarter de-interlacing methods on the VLC wiki (none are available in MAX though).
it's a texture processing effect, so you use jit.gl.slab.
also, you can enable output_texture on the jit.grab for improved performance, since you're going straight into GL land with the deinterlace shader.