In the meantime I tried. The ZyXEL VMG1312-B30A does not offer SNMP access (edit: at least not with my firmware, the "SNMP" config screen mentioned in the manual is simply missing, and there is no snmpd running or even to be found anywhere in the file system.)
And even if it did, the snmpwalk output of the related (but not identical) "B10A" variant suggests that it would only expose router-related statistics, not DSL modem statistic.
Also, there is a "dslstats" software to be used with Broadcom modems which ... does exactly what I do: retrieve statistics via telnet. However, the website has a compability list with a bunch of modems where my approach, inelegant as it is, might work too.