I'd possibly try this, as an experiment.
Mild cheddar melts better than mature or vintage, so I'd be tempted to try extract the best from both.
Start with butter & milk, a dash of mustard powder, salt/pepper & add very mild grated cheddar.
Heat & stir until it will combine smoothly.
Mix in your part-cooked pasta.
Add a good handful or two of vintage cheddar [the cracklier the better], cut into small cubes or broken into 'rustic chunks' - stir very briefly, don't wait for it to melt.
Oven bake - additional grated vintage cheddar, pepper &/or breadcrumbs on top depending on whether you like that crunchy top.
The mild cheddar will give you a hint of 'sauciness' even without the roux, the vintage will give you a good hit of flavour in bursts. The butter will separate out a bit - like it does by day 2 anyway - but I don't think it would spoil the presentation.
Bear in mind that even without the roux base, the liquid will be pulled into the pasta as it cooks, so start wetter than you require in your end result.