flatten-merge history

Hi,

suppose the following history

     P - - - Q - - - - - R       <-extern

A -- - B - - - C - D - - - E   <-master
          \           \
           M ...       \         <-b1
                        \
                         W ...   <-b2


Note that master and extern do not have a common parent. Both histories 
are 'distinct', they do not share common files, so there can't be any 
merge conflicts. What i want to achieve is this history:

     P - - - Q - - - - - R       <-extern

A -P'- B'- Q'- C'- D'- R'- E'  <-master
          \           \
           M'...       \         <-b1
                        \
                         W'...   <-b2

The two histories should be merged in chronological order.
So while master reflects P-Q-R, b2 should only reflect P-Q and b1 should 
only reflect P.

All my current attempts (surgery with git replace or interactive rebase 
combined with merging) were not successfull.

Any ideas?

Best regards
Henning