Banksia oblongifolia, to my mind, would have a better chance than dryandra. As your pots are concrete, and may leach lime (Banksias hate lime), yeah, I'd definately coat them. A coating or two of shellac'll do the job. As for water, in Sydney (or anywhere for that matter) after the plant(s) have settled in I'd rely on the finger test, ie. whenever the pot feels dry down to finger's depth, I'd water. Also, depending on the size of the pot(s), I'd keep an eye on the drainage holes, if roots begin to protrude I'd either move to a larger pot or when changing the soil consider doing a root prune while disturbing the roots as little as possibly. eg. if the pot is full of roots and it's not practible to move to a larger pot, trim about one inch all around so as to allow fresh soil (native potting mix) to take up the spaces. Now and again, about every 3 to six months, I'd sprinkle a handfull or so of native slow release fertiliser over the top. Mind you, unless you want bonsaied banksias, eventually, you'll have to plant them out.

Funnelweb, eastern Banksias may dislike lime (such as B. obl), but species from WA prefer lime and a method for success in eastern gardens is to grow them over buried limestone or cement pieces - in sand of course. Now summer rains in the east for the WA forms is another thing.

Well, Yeah, Look, I agree, but: I have planted here in the most eastern part of the continent (a few kilometres north of Byron Bay), Banksia coccinea, and, despite all the advice to the contrary, not only has it thrived, and still is during this summer of high humity, it has flowered (see the picture I posted, or had posted by a friend - probably now on the second or third page). Recommendations for WA Banksias is that they grow on a subbasis of limestone. Now, I tried that on my first coccinea and it failed! This second, and, so far surviving one, I bought from Bunnings and just stuck in the ground, watered in as per expert advice for establishing natives, and forgot it - survive or die, I thought, I was experimenting. It is Banksia coccinea 'Waites Flame'. I dunno if the Waites' propagators (in S.A.) interferred with the genetic code or whatever, but mine, in well drained volcanic (red) soil is thriving even despite our summer humidity. Maybe it's global warming! Maybe it'll die next year! I dunno, I'll see how long it survives.