There is no special meaning. As one of the linked entries says:
I guess these might be originally coined as a wordplay so that it sounds like a person's name. ～はち/～ぱち (八) was a typical male name-suffix in the Edo period (eg 甚八, 新八). Since ancient times Japanese people often created meaningless phrases just because they sound nice (examples).
I don't think でこっぱち ("forehead") can serve as a na-adjective. For やけっぱち and うそっぱち, it's technically possible to add さ, but it's not common because they already have intense meanings (you don't hear 最高さ often, either).