How to modify parser in PostgreSQL to handle new keyword and parse it

I am working on implementing Selectivity hints feature in PostgreSQL 9.3.4.
I am working on this only for using it in my academic research. I have
decided to give selectivity information per relation as part of query like
shown below.

select * from lineitem, orders where l_extendedprice <=2400 and l_orderkey
= o_orderkey selectivity(lineitem, 0.3) selectivity(orders, 0.7);

I tried separating the selectivity hint portion of the query before
Postgres parses query. But it becomes very clumsy. I am thinking Postgres
parser modification to handle this case could be complex that is why I did
not get into Postgres grammar and parser. How should I take this
selectivity hints separate from a normal query and populate in my data
structures?