if you already have the line "set cmdline" where it is setting other options then you can just ad the rel_path=*directory of choiceXXX* part in to the string of stuff. this + gen y's dual boot makes this a real deal for me
edit:
here is my startup.txt just for reference
Code:
set ramsize 0x10000000
set ramaddr 0x10000000
set mtype 2292
set KERNEL zImage
set initrd initrd.gz
set cmdline "lcd.density=240 msmts_calib=0x9f.0x39a.0x35c.0x78 7x00.a11=500 msmvkeyb_toggle=off pmem.extra=1 gsensor_axis=2,1,3 pm.sleep_mode=1 hw3d.force=1 physkeyboard=rhod400 htc_hw.force_cdma=1 rel_path=android/andboot "
boot