Yep, that's what I did, as the car was already painted etc I made up some ally cover plates to cover the holes rather than welding them shut. The hole needs to be cut about halfway up the pillar just before the "bump", otherwise you are tangling with the rear door hinges.
Here's a closeup from an older photo.
Here's what I have found about seatbelts and re-fitting with new webbing.
1. New webbing is dirt cheap $65 for 100 metres, the seller does NOT want to know what you are using it for.
2. The Upholsterer/Aircraft fitter in WA that does the work is not keen on divulging their "secret" of why they are approved, only that they will accept more "collection" points throughout Australia. Even then the boss did not return my calls.
As I understand Stamps in Melbourne are one such "collection" point.
3. Dept of Transport SA say that you need to be certified by Standards Australia to renew seatbelt webbing
4. Standards Australia have no standard for renewing the webbing on old seats. Nor any certification process.
5. Klippan in Melbourne(?) are a possible lead for replacing webbing on old seats.
6. There is supposed to be a company in QLD that test rewebbed belts to Australian Standards, but I have yet to find it.
7. Despite having two trimmers licensed to renew webbing, a prominent SA trimmer won't touch old seatbelts (liability nightmare). They didn't say much about the licensing process either.
8. Various people within the trimming and automotive industry seemed to have varying ideas about legalities of older seatbelts, ranging from "it's OK" to "it's jail time if you touch them".
