How to change/replace an in-memory palloced tuple- tuple data

Hi there ,             i have this scenario :

- postgresql 7.5 ( with modified heap_*  functions to retrieve data
from another dbms)
- the tuple's data is allocated in memory via palloc   (the  tuple table slot and the heap tuple hold a pointer to the
data and not to the buffer pages nor hard disk data pointers)
- the changes (by now) gonna be implemented in  sequential scans. 

Having these , I need to modify the tuple after the ExecQual , and
after the projection so i can save memory , just changing the tuple to
hold a "hash" or compressed data , and decompress / or restore the
tuple data  in the ExecSelect phase.

I tried to do this using  the heap_modifytuple function , and
returning a "newSlot  with a newTuple" instead the "resultSlot" but as
i suppose im missing something  , can anyone help me ? or give me a
clue....

Thanks in advance..
Weimi.. 
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