I agree with the need to have custom fields and have suggested this to Adobe. The best implementation is in Extensis Portfolio where you can choose the type of the field (eg date, numbers with x decimals, drop down list, memo etc).
Right now you have 2 choices in LR - Keywords or Collections. In each case you can build up a hierarchical structure. If you use keywords, you might name the base keyword with an underscore or other character, so they sort before regular keywords. Also note that any keyword can be defined so it doesn't export, which lets you prevent those internal categorizations getting into files you send out. You have to be careful with these though - make sure every level of the hierarchy is set not to export. If you use Collections, this isn't currently an issue as export files don't contain Collection info. With keywords, you can define the list externally in a text file and then import them.
John