Have you tried experimenting with the options already available in DXTBmp?

The Scaling and Processing filters available (below the Alpha thumbnail) let you use different scaling algorithms and also image data processing (darken, lighten etc).

While these are primarily mipmap functions some of them may work on the main image as well.

My problem is that I use the nVidia DDS compression library to do the actual compression so I am limited to the options available in that. Long story but for various technical and programming reasons I have been unable to use the later versions of this library (since they switched it to .NET) and am stuck with a version several years old.

You could possibly improve things by pre-dithering the image to 16 bit colours before compressing (PaintShopPro can do this pretty well). I could add code fairly easily to DXTBmp to reduce to 16 bit before compressing but without some clever dithering code as well you would probably start to get nasty banding effects. Certainly something I can look in to.