Well, unfortunately that is not that easy. DHCP option comes as raw byte array inside string type, not as hex string representation.
Format of option 212 is like this (values are a bit changed, but should be good enough to get the idea):
:local v212 "\0E\26\20\01\22\02\F0\00\00\00\00\00\00\00\00\00\00\00\44\2B\FF\FE"
First byte is v4 mask length, second byte v6 prefix length. Then 16 bytes of ipv6 prefix. Then 4 bytes of 6to4 6rd tunnel endpoint.
The best that I came up with is something below. Ugly, to my taste, but good enough starting point:
:global string2array do={
:local ascii "\00\01\02\03\04\05\06\07\08\09\0A\0B\0C\0D\0E\0F\
\10\11\12\13\14\15\16\17\18\19\1A\1B\1C\1D\1E\1F\
\20\21\22\23\24\25\26\27\28\29\2A\2B\2C\2D\2E\2F\
\30\31\32\33\34\35\36\37\38\39\3A\3B\3C\3D\3E\3F\
\40\41\42\43\44\45\46\47\48\49\4A\4B\4C\4D\4E\4F\
\50\51\52\53\54\55\56\57\58\59\5A\5B\5C\5D\5E\5F\
\60\61\62\63\64\65\66\67\68\69\6A\6B\6C\6D\6E\6F\
\70\71\72\73\74\75\76\77\78\79\7A\7B\7C\7D\7E\7F\
\80\81\82\83\84\85\86\87\88\89\8A\8B\8C\8D\8E\8F\
\90\91\92\93\94\95\96\97\98\99\9A\9B\9C\9D\9E\9F\
\A0\A1\A2\A3\A4\A5\A6\A7\A8\A9\AA\AB\AC\AD\AE\AF\
\B0\B1\B2\B3\B4\B5\B6\B7\B8\B9\BA\BB\BC\BD\BE\BF\
\C0\C1\C2\C3\C4\C5\C6\C7\C8\C9\CA\CB\CC\CD\CE\CF\
\D0\D1\D2\D3\D4\D5\D6\D7\D8\D9\DA\DB\DC\DD\DE\DF\
\E0\E1\E2\E3\E4\E5\E6\E7\E8\E9\EA\EB\EC\ED\EE\EF\
\F0\F1\F2\F3\F4\F5\F6\F7\F8\F9\FA\FB\FC\FD\FE\FF"
:local string $1
:if (([:typeof $string] != "str") or ($string = "")) do={ :return ({}) }
:local lenstr [:len $string]
:local res ({})
:for pos from=0 to=($lenstr - 1) do={
:local ord ([:find $ascii [:pick $string $pos ($pos + 1)] -1])
:set res ($res, $ord)
}
:return $res
}
:local v212 "\0E\26\20\01\22\02\F0\00\00\00\00\00\00\00\00\00\00\00\44\2B\FF\FE"
:put [$string2array $v212]