(Mis)using the PostgreSQL planner to get estimated row counts

I'm trying to write a PostgreSQL extension to estimate the number of  
rows returned by a SELECT statement. Ideally, it'd be invoked along  
the lines of "SELECT estimate_row_count('SELECT foo FROM bar INNER  
JOIN baz ON (id) WHERE a=b');", and would be useful for estimating  
the number of pages in a search result, for example.

I've got as far as figuring out how to get an estimated row count  
from a Node object for a query (mostly by looking at the code for  
EXPLAIN), but there I'm a bit mired - I can't figure out how to take  
a string representing an SQL statement and parse it into a Node  
object I can feed to the planner.

So, a couple of questions:
1) Can anyone suggest where I should look in the source for the  
requisite functions for parsing an SQL string into a Node I can feed  
to the planner so I can get an estimated row count?
2) Is this something that's reasonable to do in the first place? I'm  
not sure if extensions are supposed to be allowed to delve into  
PostgreSQL's internals this much.
3) Are there any other gotchas around this area? For example, there's  
a lot going on with Snapshots and ActiveSnapshot that I really don't  
have any idea about.

-Nick Johnson