Non-trivial rewriting sql query

Jan,

we're thinking about possibility to integrate our full-text search
into postgres. There are several problems we should thinking about
but for now we have a question about rewrite system.

Is't possible to rewrite SQL query and execute it. Currently we build
sql query outside of postgres using perl.

Let's consider some simple example:

create table tst ( a int4, b int4, c int4);
   select * from tst where a=2 and c=0;

we need something like:
   select * from tst where str and c=0;

where str is a string resulting by call ourfunc(table.a, 2)
and looks like  'b=2*2 or b=(2-1)'

i.e. instead of original select we need to execute rewritten select
   select * from tst where (b=2*2 or b=(2-1)) and c=0;

in other words we need to know is't possible to recognise
(operator, field,table) and rewrite part of sql by
result of calling of ourfunc().

We're not sure if it's a question of rewrite system though.

Any pointers where to go would be very nice.
Regards,
    Oleg