Best way to have table-like fields on userdata

[Brian McCallister <brianm@...>]

What is the best way to support something akin to table like fields  
on userdata, so you can have:

foo.status = 201
foo:puts("hello world")

where foo is a userdata?

Using the metatable for adding methods as described in PiL works well  
for adding methods to it, but providing an __index on the metatable  
causes __index to be invoked rather than looking up the value in the  
metatable. The only ~obvious way I see is to maintain a hash of  
values in the userdata and do all lookups there, but it would be  
handy to be able to let Lua do that for me. This could be done with  
an upvalue on __index, but doing the __index against an upvalue seems  
like a hack.

Anyone have a good solution to this?

Thanks!

-Brian