General cogitations on seatpost versus mudguard mounting: I was briefly considering replacing the dodgy Secula with a seatpost mounted one, but there isn't much vertical gap between top of mudguard and bottom of my capacious saddlebag. Enough to put a light in but the angle of visibility would be a bit narrow. Below the mudguard seems to be asking for obscuration from the side. And there's the problem of leaning the bike against things. Against this, a mudguard mounted light can be a bit low to be visible in traffic and is also vulnerable to knocks and bashes. And some models have a bolt which protrudes a long way on the inside of the mudguard, necessitating some chopping or cutting in an awkward space. A light mounted on the saddlebag works well if it can be kept reasonably still – a support such as a Bagman is useful in this respect – but, unless the bag is permanently fixed to the bike, it's not really practical for a dynamo light. Though I suppose some sort of block connector could be included in the wiring near the light itself. Obviously you'd need a back up plan for riding without the bag in that case.
The answer, clearly, is to put your back light on a rack. Unfortunately not all bikes suit this. Or to put it on the seatpost and be luggage-free.
