Exactly. [ka broda] really doesn't have that much to do with "-ness" at all, by itself. What [ka broda] means exactly, really depends on the selbri relating it.
[xy. silnyselvu'i] -> [xy. silna se vrusi] -> [xy. se vrusi lo ka silna] -> [lo ka silna cu vrusi xy.] "X tastes salty/like salt."
From what I gather about [ka] and the way it's used, it makes a secondary claim. In this case, the secondary claim is that [xy] *is* a silna1, in a way. In particular, it's silna1 such that it *tastes* like silna1 tastes, i.e. salty.
Making up gismu isn't done lightly, and making up zi'evla/fu'ivla for things that have short lujvo equivalents is a "waste" (hardly, considering zi'evla-space is unlimited) of zi'evla-space.
All in all, [broda se vrusi] produces a [x1 tastes like <lo broda>] type of selbri; I'd say that's the best solution, when such a broda exists. Otherwise, we have to resort to fu'ivla.