ACR CREW DSNDED BELOW THE 6 DME AT 2000 FT RESTR WHILE ON APCH TO RWY 29 AT OAK.

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WE WERE FLYING A NIGHT APCH TO OAKLAND INTL ARPT; COMMUNICATING WITH APCH CTL AND GIVEN INSTRUCTIONS 'CLRED FOR THE VISUAL APCH RWY 29 MAINTAIN 2000 FT UNTIL 6 DME AND 180 KTS AS LONG AS PRACTICAL; CONTACT TWR AT THE MARKER.' THE FO WAS FLYING THE APCH USING THE ILS FOR GS INFO SINCE RWY 29 HAS NO VISUAL GS INDICATOR. AS WE APCHED 2000 FT; I SWITCHED MY HSI DISPLAY TO VOR MODE AND NOTED THAT WE WERE OUTSIDE THE 6 DME ARC. I TOLD THE FO THAT WE HAD NOT YET REACHED THE 6 DME AND THAT HE NEEDED TO MAINTAIN 2000 FT. SINCE HE WAS ESTABLISHED ON A PUBLISHED GS; AND THE RWY WAS CLRLY VISIBLE AND GETTING VERY CLOSE; HE HESITATED SLIGHTLY SINCE IT SEEMED QUITE UNREASONABLE TO ABANDON AN OTHERWISE STABLE APCH SO CLOSE TO THE RWY. BY THE TIME WE REACHED THE 6 DME WE WERE 250-300 FT BELOW OUR 2000 FT XING RESTR. THIS EVENT OCCURRED DURING A PERIOD OF HIGH RADIO TFC WHILE MULTIPLE COCKPIT DUTIES WERE BEING PERFORMED. NO CONFLICTING TFC WAS OBSERVED EITHER VISUALLY OR ON THE TCASII. WE HAD BRIEFED THE APCH NOTING THE PROX OF A SMALL ARPT NEAR THE LOC COURSE TO RWY 29. HOWEVER; NEITHER OF US WAS AWARE THAT AN ALT RESTR WOULD REQUIRE US TO ABANDON THE ILS GS; AND THEN REACQUIRE IT FROM ABOVE AT SUCH A LOW ALT. THIS LED TO SOME AMBIGUITY AS TO WHAT WAS THE PROPER PROC FOR THIS APCH. I CANNOT RECALL ANY OTHER SUCH SIT WHICH EXISTS ANYWHERE AT THE LOW ALTS INVOLVED. THE PERSPECTIVE OF THE VERY LONG RWY 29 WITH VERY FEW SURROUNDING LIGHTS ALSO GIVES THE ILLUSION OF BEING EVEN CLOSER THAN YOU ARE.