how to define fields of odt, which would not be affected by digi-signature?

Hello,

We want to achieve with OOO what MSO proposes now -- ability to
include graphical info alongside with digital signing.
MSO has fields (placeholders) for this graphical info (photo or
signature photo of the signee), which are not included when hashing
the document

But when signing digitally, probably all odt elements are hashed together.
How could I define the fields, that  need to be skipped when signing?
but be shown in, lets say, oowriter ?

If the idea is feaseble, we could build extention in ooo,
to  store those fields in separate .xml inside odt,

then signature checking would work fine
if the fields and graphical extra data are not mentioned in signatures.xml

if I am too optimistic or naive,
pleas, tell, what is missing most,
and when could I/we expect it

Thank You in advance

pps.: I also asked this q  ooo devs, but no response
http://www.openoffice.org/servlets/ReadMsg?list=dev&msgNo=22525

-- 
Jurgis Pralgauskis
Don't worry, be happy and make things better ;)