creating index on changed field type

Hello,the subject is obscure, so I will try to explain. I would like to 
develop index based on text field (or tsvector stolen from tsearch2), 
but containing different type (for example cstring, varchar,etc.) in 
order to tokenize the original field. I would like to use postgresql 
btree implementation, but AFAICS I can not do it. Example:

CREATE TABLE test (id int, mytext text);
CREATE INDEX myindex on test USING myindex (mytext) ;
INSERT INTO test VALUES(1,'this is my first text');

In index I do not want to keep whole phrase, but words derived from it
('this', 'is', 'my', 'first', 'text').

My idea was to create functions mybtgettuple, mybtinsert, mybtbeginscan , mybtrescan and so on. And in every case
ignoringoriginal
 
IndexTuple, and create set of new IndexTuple's (one for every term) and 
involving original functiions.

The problem is that index_create() in catalog/index.c creates everything
in system tables, especially type of index field.

Should I forget about btrees and move to GIST, or is there any hack,
which could solve my problem? Please help me.

Thanks in advance,
David

ps. maybe I should create index on TEXT field, store terms (words form 
the original field) also as TEXT type? Will it work?