Before committing to this particular solution we can consider this other option. The change is more intrusive, but maybe it will make things more organized:

• Use rgba[4] instead of rgb[3] and alpha storing the color, this way is more clear than the alpha is always associated with the color. It should be accessed using BKE_brush_color_get() only when used for painting tools.
• Rename the sculpt_strength of this patch to strength. This way the strength property can be used by displacement tools or by color tools that need strength but not alpha associated with a color (blur, smear...)

To change to RGBA instead to RGB could have impact in GPencil because the Alpha is not used and depends of other parameters. To keep GPencil running we would need to have in RNA a prop with 3 (RGB) to avoid the UI displays the Alpha channel selector. Also need some small adjustment in the paint code, but nothing big.

I'm talking of this prop:

I don't think RGBA would be convenient for palettes? Don't you usually want to keep alpha the same and only change RGB? To me it seems best to keep RGB and alpha separated, I think in general users will edit them separately.

Renaming sculpt_strength to strength is fine with me.