Haven’t tested this thoroughly, but here is how you can get the same functionality as read_default with a custom method:
require 'win32/registry'
def custom_read_default(section, variable, default = nil)
keyname = "Software\\SketchUp\\SketchUp 2015\\#{section}"
Win32::Registry::HKEY_CURRENT_USER.open(keyname) do |reg|
return reg.map{|key| key}.include?(variable) ? reg[variable] : default
end
end