I want to basically have an "id" type in Oracle.
An example which would satisfy my needs would be a 32/64 bit integer where NULL is represented as -INT_MIN.
I don't need this to be variable length (like NUMBER). I considered using RAW but I don't believe there is an implicit conversion from RAW to integers.
It seems an option is BINARY_DOUBLE. Is this the best approach?
Use NUMBER(10, 0), which is equivalent to NUMBER(10). BINARY_DOUBLE is a floating-point representation, not an integer. I figure, don't second-guess the database, unless you have a specific reason to do so. You may be talking yourself out of behind the scenes data optimizations that the CBO can do (though I don't know if there are any). You say you want an integer, so use an integer.
And why would you want to represent NULL as -INT_MIN? Represent NULL as NULL.
Not sure of your entire requirement, but yes it appears you're correct:
http://download.oracle.com/docs/cd/B19306_01/server.102/b14220/datatype.htm#i16209
