you need HowlingWolf's unlocked SP08 for G.INP, however BT may have suspended the rollout of G.INP for the moment. Either way it is recommended you upgrade to SP08 as it bitswaps better.
DSLStats would need a major re-write to be able to run on Smoothwall, as it requires a GUI, which Smoothwall does not have. The way I get around it is I have the HG612 LAN1 plugged into SW RED, and LAN2 has an IP on my GREEN subnet and is plugged directly into my GREEN switch. I then have a Raspberry Pi plugged in on my GREEN network running DSLStats and uploading to MWDS.
You could write your own scripts to gather the telnet data from the HG612 on SWE, but you'd have to figure out some way of storing the data, then you'd have to analyse it yourself.