Showed up in a list of Ryo Okumoto’s gear on the Spock’s Beard official website.

In an interview featured on Claude Schnell's official website, he discusses using the Mellotron M400, highlighting its role in his performances.

Geoff Nicholls is confirmed to have used a Mellotron M400, as evidenced by a photo available on Equipboard.

Simonetti is credited with using an OB-X, Roland System 100, Minimoog, CS-80, Prophet 5 & Mellotron

Bill has played Mellotron on Circulatory System's Signal Morning, Casper & The Cookies' Oh!, and on both Sunshine Fix albums

Jürgen Fritz is credited with playing a Mellotron M400 in the liner notes of Michael Holm's album "Stories," according to Discogs.

Erik Norlander is confirmed to use the Mellotron M400, as detailed in the Planet Mellotron Album Reviews: Lana Lane. The source notes several instances of Norlander's use of the Mellotron, including credited tracks with choir, flute, and string parts in his keyboard work on albums associated with Lana Lane.

Simon House often used a Mellotron during his years with Hawkwind, as evidenced by the credits for Hall of The Mountain Grill.

Shown on the cover of Closer to Doom

"The end of the 1970s saw a huge rise in the development of synthesisers, and in making traditional instruments like pianos and organs more transportable and able to be amplified well. In the early days Split Enz always used to need an acoustic piano at every gig, which was a pain for us (amplification without feedback was always an issue) and promoters alike, but with the advent of Yamaha’s CP-70, that all changed. Synthesisers had also become polyphonic and memory space to store your own sounds was also a new development. The 'space capsule', as the guys called it, was seven or eight different keyboards, including a CP-80 Yamaha (full 88-note piano), a CS-80 and Prophet 5 (both early polysynths), a CP-30 electric piano, a Mellotron for 'real' string and choir sounds, a string synth, and a monophonic synth such as an EML 500 or Cat Octave … Oh, and I bought the first Korg Vocoder to arrive in New Zealand, which was fun! As you can imagine, transporting the sheer weight and volume of all this gear became prohibitive, so we replicated the setup in the US. Unsure what happened to all that gear when the band broke up!"

At 2:03 in this live video for the song "Bedside Manners Are Extra" David can be seen using a Mellotron M400 (I'm currently trying to find info about what tape set he had loaded!)

In the YouTube documentary "The Secret Keyboardist of Robert Plant & Black Sabbath: Jezz Woodruffe" by JCM - Led Zeppelin Stories and more, it is confirmed at 5 minutes and 19 seconds that Gerald Woodruffe used the Mellotron M400 on his solo album "Opposite Directions," as all the keyboards utilized for the album are displayed.