So if you are looking for cadenzas to viola concertos different from the usual ones, this is the page you need. If you need more information, read this What is a cadenza? page, with some video examples.
Here are some viola concertos cadenzas composed by the great viola virtuoso Michael Kugel, my favourite viola player.
Maestro Kugel composed cadenzas for the most famous viola concertos like Stamitz viola concerto and Hoffmeister's (you can see the first page here below) and also for some lesser known concertos.
What I find more interesting in the book rather in other cadenzas is that they have been written by a performer rather than by a publishing house editor, as it is in most cases with cadenzas. This what it used to be in the old times: performers would compose or improvise their own cadenzas.
He wrote a cadenza for each of the 3 movements of the following viola concertos:
Find here his viola cadenzas book that also contains a Duo-Sonata he composed for violin and viola.
On the maestro publisher's website you can also find more viola music by him and a book he wrote on Shostakovich Viola sonata and Bartok Viola concerto. I wrote a page on this website about his analysis of Bartok Viola concerto.
In case you wonder how the cadenza relates to the not so rare viola concertos, luckily, there are several cadenzas that have been composed for them. There is even a cadenza that is a composition with its own life, not connected to any other work. See in this page, more cadenzas to viola concertos a list of them with links for those who'd like to buy and perform them.