I am going through the process of setting up a home anodizing line, but have not yet started working with it, so this is just speculation on my part.
When you say "solder", what aluminum soldering material(s) are you talking about using? My concern would be with the possible reaction of the solder with the acid in the anodizing tank (either with corrosion or contamination of the bath). I have never worked with soldering to aluminum (or the special solders that are sold for that purpose), and so I also don't know whether electrical conductivity through the layer of solder would be an issue.
If the back of the part is not important cosmetically, would it be simpler to drill a blind hole there for attachment to the anodizing rack, rather than soldering or welding or some similar procedure?