After some discussion, it seems there are many differences between a BSON::Regexp::Raw type, and a user might want to work explicitly with those types.

class User
  include Mongoid::Document

  field :reg, type: BSON::Regexp::Raw
end

u = User.create!(reg: BSON::Regexp::Raw.new("hello"))
p u.reg
# => #<BSON::Regexp::Raw:0x00007fcfa8317b10 @pattern="hello", @options="", @compiled=/hello/>
u = User.find(u.id)
p u.reg
# => #<BSON::Regexp::Raw:0x00007fcfad2825d8 @pattern="hello", @options="">

This already works. However this ticket could also be used to create a mongoize and demongoize method so that a string/regex assigned to this field can be coerced to a BSON::Regexp::Raw type.