I am very familiar with the packages and structure as I have used KDE since the old days, but without looking into it too deeply . . .
I assume that htdig is only necessary if you want to install trinity-base/khelpcenter.
The trinity-base/tdebase-meta ebuild has trinity-base/tdebase-meta as rdepend.
You might be able to maintain your own trinity-base/tdebase-meta ebuild, and comment out
the line
or rather than installing the meta package, create your own trinity meta
package set eliminating khelpcenter but keeping everything else in the ebuild.
Instead of installing trinity-base/tdebase-meta, install your package set instead.