For the sake of the less esoterically informed, I thought I would point out most of the above mentioned composers were being reasonable and practical in composing for these now 'exotic' instruments.
There are often extra-musical requirements (light organ being one) required by composers of Opera: Horses (Wagner, at least), Elephants (Verdi), etc. Can we assume the question excludes non-musical elements, including light-organ?
Most of the instruments listed here are later substituted due to their 'retirement' from general usage, or if still available but obscure are often substituted as a matter of economy.
Ravel's Tzigane accompaniment was for a readily available Lutheal, granted, new and exotic at the time of composing but readily available. The new instrument probably was a stimulus for the piece. (For this delightful news - to me - of the existence of the Lutheal, thank you Mr. Selick!)
Schubert's Arpeggione was likely commissioned by a friend who was a virtuoso of the new instrument. Again, not a composer writing for some obscure, idealized instrument.
Scriabin's color organ, a silent electronic machine which operated a range of lights via a piano / organ style keyboard, existed when he was asking for this synesthetically conceived instrument. The fact it was not a massive pipe-organ like installation makes this composer's desire to include it not impractical. (The work was performed with the 'organ' in post WW1 England.)
Wagner would most certainly not be one to compose for obscure instruments which then needed to be tracked down and players netted in ~ he too was composing for readily available instruments and reliable players thereof.
Stravinsky called for all-male choristers in his Symphony of Psalms. The piece is performed most often with adult females singing the treble roles. The Stravinsky conducted recording on Columbia is thus, and it is a good guess if Stravinsky had asked for / demanded child male treble choristers, he would have gotten them.
Tchaikovsky's Dance of the Sugar Plum Fairies, now known for its use of Celeste, was first conceived of for Glass Harmonica. During a visit in Paris while in the Pleyel showrooms, Tchaikovsky was shown the brand-spanking new Celeste which was still under wraps. He was very taken with it and he left the Pleyel showrooms with an exclusive agreement: Pleyel would not make the instrument public or available until after its debut in the world premier of the Nutcracker Ballet.
Saint-Saens originally composed the Carnival of the Animals for Glass Harmonica (especially the melody for the 'Goldfish' movement) : a Celeste is most often substituted. (because Celeste was 'the new rage' or because the new instrument's availablity made it much more pragmatic than a Glass Harmonica?)
Stravinsky's Threni calls for a Contrabass Sarrusophone. This is a rare critter. But certainly, the composer knew one would be available, and along with the certainty of being Stravinsky, he could rely on an orchestra being willing to pay the double or extra player fee.
Resphigi composing those songs and operas for singers with rare voice qualities and freakish range capabilities? He was composing for singers he knew.
Ned Rorem composing those soprano songs, the range of which goes up into the Minnie-Ripperton territory upper stratosphere ~ a singer he knew.
p.b.
EditADD / P.s. It seems we have all overlooked a blazing flash of the obvious: real field artillery Cannon and real, large, cast Church Bells in that Tchaikovsky chestnut of chestnuts, The 1812 Overture!