A self reply;
rootfs on fxp0 did work with 200812310000Z (5.99.5) and has gone wrecked sometime in Jan and keeps broken through Feb and today.
For data points, it looks like (only) part of fxp family HW suffer from no DHCP request symdrome. I found one fxp works correctly (from 1231 thru 0227). P3-933 CompaqDeskPro SFF Intel 82801BA LAN Controller / 2449.8086 Toru Nishimura / ALKYL Technology