At the end of Harvey's Revised English Grammar (1878) is a system which looks similar to R-K diagrams, but which is greatly simplified, in what appears to be unfortunate ways. Visually, note the absence of R-K's slanting lines for modifiers. Also, notice that the various modifiers are, in many cases, not clearly separated.

Sample diagrams for complex sentences, showing content clauses, infinitives, etc. (pp. 268-269)

The last page of this volume contains a plug for another book of diagrams by the same publisher: Irish's Grammar and Analysis Made Easy and Attractive by Diagrams. Given the nature of the diagrams in Harvey's Grammar, I suspect that this diagram system was cribbed from Reed and Kellogg, with enough changes to avoid copyright problems.

In E. J. Hoenshel, Hoenshel's Advanced Grammar (1899), there is a similar system that appears also to be an adaptation of Reed-Kellogg. Unlike Reed and Kellogg, the author does distinguish predicate nominatives from predicate adjectives, separating them from the verb by using an '=' for the former and a '-' for the latter.

Sample diagrams of relative clauses. (pp. 286-287)
Sample diagrams of adverbial clauses. (pp. 288-289)