This is what I expected the problem with using it would be. Thanks for the answer.

I didn't have any word in mind, it's just something that I thought of when I was learning about ーさ. I did expect that it wouldn't be used, but the construct hasn't appeared nearly enough in order for me to be able to get a feel for it. As Earthliŋ says, the question boils down to a semantic one: why not? I only have two semesters behind me, so I can hardly call myself experienced enough to understand how the Japanese say things. Thanks for taking the time to write an answer.

@YangMuye Sorry for the late reply, but I was thinking more along the lines of the former: おいしそうさ

@YangMuye I figured that was the case, thanks for clarifying.

@YangMuye Since そうな is an adjective - e.g. as in here: これはおいしそうなケーキです- does it make any sense if we modified it with ～さ (as in 大きい ->　大きさ)? That's the gist of it. Personally, I'm a bit doubtful about this being the case. However, if not, that leads to my follow-up question: what other kinds of adjectives cannot be modified this way? Hope this cleared it up a bit. I'll look into らしさ, thanks.