I think if you use a solid brass rod as an "everyday" ramrod, to be carried in the ramrod pipes, it will add considerable weight to your firearm. More weight to carry, and more weight to hold up if you are shooting offhand. It may also want to slide out if you should tip the muzzle down, due to its weight. Furthermore, the forward lock bolt on flintlock rifles sometimes impinges on the ramrod channel and the ramrod itself. The traditional "fix" for this problem is to taper the distal end of the rod. Putting that taper on a brass rod may be beyond the capabilities of many, including myself. With a wood rod, it's no problem.
If I were to use a brass rod, I think I would leave the surface smooth. You can always darken it with Brass Black, or just wipe it with dirty cleaning patches. The sulphur in black powder and black powder fouling reacts with brass and turns the surface black.
Delrin rods have been around for quite a while. Many people criticize them because of their flexibility, but they aren't going to flex inside the gun barrel, and it's good practice to ram your load with multiple short strokes rather than one long "swoop," anyway. The Delrin rods I have seen tend to be gently curved rather than straight, but they straighten out in the rifle bore and the curve in combination with the rod's natural springiness helps it stay in place, in the ramrod pipes.
Wood rods work fine, when used properly and made of the right type of wood with the right grain structure. Wood rods are traditional for most trade and sporting guns and rifles. Straight-grained hickory would be my choice for an everyday ramrod.
Please don't take any of this as criticism of your idea. Brass rods have their place, and they can work very well for their intended purpose. However, you requested our thoughts on the use of brass for an "everyday" ramrod, and those are mine.
Best regards,
Notchy Bob