SQL Either Or Statements In Third-party Software

I'm in a bit of a pickle with some SQL syntax, and while my Oracle-fu is weak, an associate with SQL skills is also stuck. I am developing a database for my department that is backed by Oracle Enterprise 11g and the front end is ChemAxon (basically, chemistry database software). The short of it is that I have one large table with every compound from different groups. Each row has a flag (column PI) on it that indicates which group it belongs to (let us say 'Smith' and 'Jones' for example). The software can apply row-level filtering which will basically only show the rows that a particular username are allowed to see.

I have a FLAG_TABLE table, which contains two columns: USERNAME and FLAG. An example set up is this:

USERNAME FLAG
------------- ----------
smith_minion Smith
jones_minion Jones
jones_minion Smith

The software automatically applies a SQL filter that begins with:

SELECT DISTINCT CHEMAXON.STRUCTURES."ID" FROM CHEMAXON.STRUCTURES WHERE

I can set up filtering to work dandy, such that when smith_minion logs in, he can only see rows with the Smith flag (or jones_minion can see both Jones and Smith) by using the filter:

STRUCTURES.PI IS NULL or STRUCTURES.PI IN(SELECT ALL FLAG from FLAG_TABLE where USERNAME = '__IJC_USERNAME__')

('__IJC_USERNAME__' is how the third party software passes the logged in username into SQL)

But we have a new problem: there also exists a master user (chemaxon) who needs to see every row no matter what the flag. The row filtering is applied no matter who logs on, so we need to set up the SQL filter to basically say "If chemaxon, then select all rows, otherwise, select rows based on the username". This is proving a problem as the select statement must be prefaced with SELECT DISTINCT CHEMAXON.STRUCTURES."ID" FROM CHEMAXON.STRUCTURES WHERE.

I've tried using DECODE in a few capacities, but I am always thwarted. My last attempt was:

STRUCTURES.PI IS NULL or STRUCTURES.PI IN (DECODE('__IJC_USERNAME__','chemaxon',(!='something'),(SELECT ALL FLAG from FLAG_TABLE where USERNAME = __IJC_USERNAME__));

But this throws a ORA-00936 missing expression error, with a * under the != portion (I test it by replacing '__IJC_USERNAME__' with 'CHEMAXON').