No, I meant TSOP. This is "performer sort order" as defined in the v2.4.0 standard. TPOS ("part of set") is defined as being native to both 2.3.0 and 2.4.0, and most often used for the disc number in a multidisc set.
I think v2.3 will be fine. I was confused by the id3 specifications and thought that the "native" v2.4 tags could only be used in v2.4 format tags. If they're used in v2.3 tags then my application still recognizes them.