Yes, pre-demosaic noise reduction exists, but it’s uncommon and not always better in practice.

From the answers provided:

• Dark frame subtraction is one established pre-demosaic method for removing fixed/pattern noise.
• darktable includes a raw denoise module that works before demosaicing.
• Some demosaicing methods, such as LMMSE and IGV, are designed to cope better with noisy RAW data.
• Most RAW converters do noise reduction after demosaicing but before sharpening.

So your reasoning is broadly sound: working on the original sensel data can be advantageous because demosaicing can spread noise between channels. However, random noise reduction before demosaicing is not standard, partly because demosaicing and denoising are closely linked problems and often work best when considered together.

If you want the most control, one answer suggests extracting RAW data with dcraw and treating reconstruction as a custom inverse-problem workflow, but that is more technical than a typical editing solution.

In short: yes, pre-demosaic denoising is possible, but for most photographers the practical options are limited, and standard post-demosaic RAW noise reduction is usually what software provides.