merge-join for domain with underlying type text

Hi Folks,

If I have a domain defined with an underlying type of "text" or 
"varchar," and I need to FULL OUTER JOIN two tables based upon the 
similarity in these fields, I am getting:

ERROR:  FULL JOIN is only supported with mergejoinable join conditions

I have looked at the docs and it seems that it's a matter of making the 
comparison operators for the domain type (as per SQL Reference :: 
"CREATE OPERATOR" section).  However, is there anything tricky about 
adding the merge-join capability to a domain?  Is there a preferred 
method?  Am I barking entirely up the wrong tree (could I, for example, 
use a casting trick?  Simply recasting the fields as type "text" 
doesn't avoid the error)?

Postgres version is 7.3.

Best,

Randall