A small nitpick: I don’t think your intersection example does what you want it to do. Perhaps there’s some obscure difference in “PER-visibility” or whatnot, but at least set-theoretically,

  LegacyFlags2 ::= INTEGER (0 | 2 ^ 4..8) -- as in the article

is exactly equivalent to

  LegacyFlags2 ::= INTEGER (0) -- only a single value allowed

as (using standard mathematical notation and making precedence explicit) {0} ∪ ({2} ∩ {4,5,6,7,8}) = {0} ∪ ∅ = {0}.