how about going into the stonewall attack from a bird's opening.  This eliminates masses of theory like the kings indian and your far more likely to encounter problems from the bird's move order.

Knightassassin,

    Are you in the Bird's Opening Lovers Group?  If "flexibility is the key" there is more out there than the Stonewall Attack.  You are trading a From's Gambit for the King's Indian Defense.  You should review the book Pawn Structure Chess by Andy Soltis regarding both the KID and the Stonewall.  If you fear the King's Indian then play 1.e4 or learn more about it from the White side.  Suprized more players have not answered 1.f4 with e4.  In which case you might want to look at the book Counter Gambits by Tim Harding.

I think it's a very good idea to play 1. f4 if you want to reach the stonewall attack and at the same time avoid theory. The from's sure is a dangerous gambit but it can't be compared to the kings indian which is a "solid" opening. Black is actually sacing a pawn in the from's and if white knows the theory well, white will just have a huge advantage with pawn up (I play from's gambit from both sides). 

The real problem arises if black playes an early g6, playing like white does against the dutch. Against this i recommend using "the polar bear system".
http://www.videochess.net/ if you want to learn more about this.

I do think playing the Stonewall Attack starting from a Bird’s move order is quite flexible, cause you can switch to a classical Bird if black adopts an anti-Stonewall, or you can play the Big Clamp formation against Sicilian lovers.