Apologies for forgetting about this topic for 6ish years.
Apparently ÖNB has (not yet digitized) about 50 works by him, chamber, operatic and otherwise, in their catalog, including "Hero und Leander", "Die Hochzeit zu Ulfosa", "Op. 2. Sonate (F-Dur) für Pianoforte u. Violine. Componirt und seinem verehrten Lehrer Herrn Professor [Anton?- my insertion] Door zugeeignet von Paul Caro. (Violine, Klavier.)", a manuscript
Symphonie Es-dur, string quartets opp.6&7, cello sonata op.42 in D minor, and other works. (The op.42 sonata is also at BNF, at Herder-Institut and in NUKAT...)
BNF has some works by him including a piano sonata in B-flat minor published by Robitschek and an F-sharp minor piano sonata op.41 at Herder. A search at Herder-Institut's library catalog reveals a reasonable amount, mostly piano pieces and lieder, but also the op.19 string quartet in D minor and the vocal score to Hero und Leander, e.g.
NUKAT (Poland) lists one digitized work by Caro:
Ein Ländler-Cyclus for piano, op.21 - Breslau: C. Becher (1898) and has about 20 items in their system not digitized. I
should probably create the category (linking it to the wishlist), and a workpage for that. Will do soon!