Imported deferred constant with an address clause legal?

[Rod Chapman <rod@praxis-cs.co.uk>]

A quick question for the language lawyers...

I have an embedded application which as a big EEPROM with lots of data
tables on it.  I want to map an Ada object to a location in the EEPROM.
Normally, I could do:

  with System.Storage_Elements;
  package My_EEPROM is
     Obj : Integer;
     for Obj'Address use System.Storage_Elements.To_Address(...);
  end My_EEPROM;

which works just fine.  BUT, what if I want to tell the compiler
(and our dataflow analyser) that this really _is_ a constant object
(it's in an EEPROM after all...)?  How about using Ada95's
trick of allowing a deferred constant to be completed by pragma
Import...

  with System.Storage_Elements;
  package My_EEPROM is
     Obj : constant Integer;
     pragma Import(Assembler, Obj);  -- Assembler? Seems OK...
     for Obj'Address use System.Storage_Elements.To_Address(...);
  end My_EEPROM;

Is that idiom legal?  Portable?  Implementaion dependent?  I can't
decide if it's reasonable to expect such a trick to work or not...

Thanks in advance,
 Rod Chapman
 Praxis Critical Systems.