Paul McCartney is confirmed to have used the RMI Electra-piano and Harpsichord (300-Series) during his time with Wings. Notably, the RMI Electra 368 can be heard on the Wings debut album "Wild Life," particularly on tracks such as the title track and "Give Ireland Back To The Irish." During the ICA rehearsals for "Give Ireland Back To The Irish," Linda McCartney is seen playing the RMI Electra 368, and Paul McCartney himself is documented using it in live performances at university campuses in the 1970s. This is supported by photographic evidence available here.

At 0:32 in this video, Dilla can be seen standing with the RMI Electra-piano in his studio.

Madlib mentions in this interview that he used the 'RMI Electra Piano'. Photos of him can also be seen with the Piano next to him.

Jean Michel Jarre includes the RMI Electra-piano and Harpsichord (300-Series) among his favorite synthesizers, as noted by Red Bull Music Academy.

[Sakamoto reaches over and plays an RMI Electra-Piano.]

One from 1970 was used for The Six Wives of Henry VIII, as is visible in this diagram of Wakeman's setup and as specified in its June 2020 Reverb.com listing.

Featured on Rick Wakeman's The Six Wives of Henry VIII

From the 1976/77 "Seconds Out" tour, a clear shot of Tony playing the RMI during "Fly On A Windshield" at 8:35

Ray Manzarek played an RMI Electra-piano 300A with The Doors on the West German television show 4-3-2-1 Hot And Sweet during their 1968 European tour. The Doors can be seen lip syncing "Hello, I Love You" in Romer Square in Frankfurt. This performance was filmed on September 13, 1968, and is available on YouTube here.

One week earlier on September 6, 1968, the Doors performed at the Roundhouse in London. At this concert, the same RMI Electra-piano 300A can be seen on stage next to Manzarek's Gibson G-101. Based on available footage from the show, it appears that the Electra-piano was never actually played.

Ray was said to dislike the RMI Electra-piano, and he quickly returned to the Gibson G-101 for all performances. Despite that, Manzarek is believed to have played an RMI Electra-piano on the song "Shaman's Blues."

In this 1975 photo, Elton John can be seen playing an RMI Electra-Piano and Harpsichord.

Steve Winwood played a RMI Electra-piano 300A with both Blind Faith and Traffic. It was Winwood's primary on-stage keyboard with Blind Faith, and he can be seen playing it at the band's famous 1969 concert in Hyde Park. The photo above comes from a recording session at Olympic Studios in London. Interestingly, I do not think the Electra-piano was played on any songs from the band’s first and only studio album. Additional photos of Winwood with Blind Faith here, here, here, here, here, and here.

With Traffic, Winwood’s Electra-piano was featured on "Empty Pages" from the 1970 album John Barleycorn Must Die.

At 5:51 the RMI can be seen next to the Hammond organ.

Used with Funkadelic, according to the following sources:

Rock's Backpages audio, June 1978, "AUDIO: Bootsy Collins (1978)", interview with Cliff White

Bootsy Collins: And Bernie is goin', you know, Bernie is goin' solo.

Cliff White: I heard he's got an album planned for— on Eris.

Collins: Yeah, we just finished his album.

White: Oh, that's great.

Collins: Yeah, so uh, he's next. And, let's see, uh... [unintelligible]

White: He's always struck me, of all the other guys, that you and George and Bernie are the three sort of kingpins, sort of...

Collins: Oh yeah, oh yeah... I mean, Bernie is def—, he's uh...

[...]

White: I don't normally like synthesizer. On a lot of records, I think it's, uh... it's just an excuse for somebody who can't really handle keyboards, so they just fiddle about, but Bernie [unintelligible]

Collins: Yeah, yeah. See he, he was doin' that type of stuff with the RMI, you know, it's a different sound [, back in the days of the Funkadelic, you know, [unintelligible]... he was gettin' some [????] sounds then, I mean, he was... you know, so when the synthesizer came out, it was, it was, you know, that was Bernie, you know what I mean?

*White: Right.

Red Bull Music Academy, 2013 Red Bull Music Academy lecture

TORSTEN SCHMIDT

Speaking of the Clavinet, who was first playing that through phasers and stuff, you or Stevie [Wonder]?

BERNIE WORRELL

I don’t know. On the track that you played with the strings, my first string, everybody thinks it’s Clavinet, but it’s the RMI piano. And Stevie had gone to, it’s called Wonderlove Music, music store in Detroit. I had gone the next day, but they told us at the store: “Stevie was at the store yesterday and got one,” and I was the second person that got one. So I don’t know. Who did what first, and “blah blah blah blah.” Who cares? We’re doing it.