You shouldn't need a plug with an idle bar dog. They were introduced when throttling of crossflow engines became possible. If you cast your mind back, the engines of that genre also had a butterfly valve over the exhaust (before the days of mufflers). The idle bar and butterly were there to keep a crossflow running when idling (idle bar?). A modern schneurle ported engine with a muffler just doesnt need an idle bar. It is more likely to cause problems than solve them. The fact that the carb is air bleed makes no difference to plug heat, it just means that the fuel is metered in a different (a bit less precise) way.

As regards the question posed, if you want to stop your clunk coming to the front then you can include a short length of brass tube in the line from the bung to the clunk. Leave a flexible length of tube at each end so the clunk still falls to the bottom of the tank so that the fuel feeds but include enough brass tube so that the pipe can't 'turn around'. Do I need to draw a picture or do you get it?