>> Have you tried the oatmeal option yourself, <<

I have in the past, though not recently. It's one of the best choices if you want a really smooth, creamy meatloaf.

>> and if so did you use "instant" or "overnight" oats or some other variation? <<

I have generally used quick oats. But really the difference is just the size. Rolled or steel-cut oats are whole flat oats. Quick oats have been broken up a little. Instant oats are almost powder and tend to be pasty. Most meatloaf recipes call for quick oats. Steel-cut oats will still be recognizable in the finished loaf, thus better suited to a chunky meatloaf. Instant oats should disappear completely, but might have the same problem with pastiness that they do as breakfast cereal. Consider mixing steel-cut or instant oats with another filler if you want to modulate the consistency. Quick oats can be used by themselves.

>>We've got some "steel cut" version that bills itself as "3 minute" but never cooked up right for a normal bowl.<<

You can either use them whole in a chunky meatloaf, or run them through a food processor to make them into quick oats.

I recommend searching online for oatmeal meatloaf recipes; there are plenty. See what other ingredients appeal to you.