You're confused...

First, ask yourself... what is a chord?

Next, ask yourself, what is a dimished chord?

If you can't answer that, then you should review it at this point.

A dimished chord consists of 1 b3 b5. Any chord can be turned into a dimished chord. ie a major chord can be turned into a dimished chord by lowering the third and the fifth of the major chord by a half step.

Now, I'll let you in on a little secret:

Barre chords are more difficult to form and more labour intensive and create left hand fatigue (ie the famous burning pain in your index finger running up your forearm) rather quickly. Anytime you can get away by not using a barre chord, do it.

You are mistaken in believing that no "true" dimished barre chord can be achieved on  the guitar. There are ONLY 3 particular SHAPES for a dimished chord on the guitar. Only 3. Each shape depends on what string the root of the dimished chord begins on.

This shape would be the easiest to form a barre chord on and shift through different inversions:

E 3

B 2

G 3

D 2

A x

E x

Now, the usual LH placement starting highest to lowest would be: 4 2 3 1

But try this flavour instead: Form a barre using your index finger starting on the 4th string, NOT the 6th string. Then simply add your 2nd finger and your 3rd finger above to form the dimished chord starting on the 4th string.

One other thing: since you said you were a pianist,  the LH fingers I gave you are on guitar, NOT piano. This is how fingering works for the guitar (if you didn't knwo already): your index is 1, your middle is 2, your ring is 3, your pinky is 4.

When you make a barre chord, a LOT of people make the mistake of using their index finger across ALL the strings, even when they don't need to be. This is a huge mistake in the economy of motion.

Hope that helps you out a bit

These guys are making my carpel tunnels hurt...