Hey Russ -
You are not alone - most of the HASL systems out there do not appear capable
of providing adequate coplanarity for microBGA and very fine-pitch.  Gold
flash of course will work fine.  Of course there is the phobia of gold
embrittlement and whether this is a problem or not will depend upon the
application and the reliability (life) aspects of it.  I've had very good
luck with OSPs and CSP and recommend you seriously consider that route.
Best regards,
Phil Zarrow
ITM, Inc.
Durham, NH
(ps: see you at SMI !?!)