I've run 55/60W up front, 12v, for long periods on one bike, and not had problems with overloading or with the drive train (but that bike is steel-gear-driven not belted).
However, that was always using original spec armature and field, ie the '6v' versions, which are heavier gauge but need more revs to cut-in (as has often been discussed). DVR2 regulator in recent years, prior to that a JG unit after they were launched way back when.
The only time I ever converted to '12v' bits (UK-made, so not crap parts in theory) also with a DVR2 - it threw the solder off at the commutator after not many months with 'just' a 45/40W halogen. With tail & speedo (no LEDs) it was being asked to deliver 80%+ plus of its current rating all the time - bit too close to the limit maybe, or I was just unlucky. That machine now runs '6v' parts at 12 and hasn't had any further problems. Lately it's been running an LED up front so the dynamo is a pretty idle beast - but that will have to change for winter if I want to see anything before I hit it as the LED version I have isn't any good.
On my A, I have had occasional belt/pulley slippage troubles over the years. The SRM kit did quite well but died in the end (lost its fit on the taper on the driving pulley after slippage). The Dyn Regulators kit that is now fitted seems bullet proof so far, even with an extra 35W of 'Hot Hands' on the 'bars and a total load therefore of around 75-80W continuous on occasion. Wider belt and easier to get on and off too owing to provision of threaded holes for extraction.
So I'd say the loads you mention shouldn't be a problem, but there's nothing quite as unpredictable as an old bike (except the memsahib of course).