https://musicforeveryone.createaforum.com/downloads/sergei-vasilenko-horn-concerto/
This recording, taken from a not very easy to find YouTube video, was uploaded when I just noticed Christopher's 2022 request in the old Art-Music Forum. A link given by a previous uploader to this piece is dead when he tried to download it.

Actually, this recording is part of a series of concerts commemorating Sergei Vasilenko. This series of musical activities also included conferences and masterclasses. The official website is here: https://vasilenko-legend.ru/
a total of 5 concerts were uploaded to YT, all features Lipetsk Symphony Orchestra, four conducted by Vladislav Bulakhov and one by Konstantin Barkov (the last one), as can be seen in the following videos:
https://www.youtube.com/watch?v=8ThYwH5GkJ4
https://www.youtube.com/watch?v=eLG8ORkbiPU
https://www.youtube.com/watch?v=Yb1mcKetAmk
https://www.youtube.com/watch?v=a-xh8h61wfw
https://www.youtube.com/watch?v=hqA4iyxjF_0
These concerts all started with Vasilenko's Epic Poem, Op. 4, and ended with his Op. 5, a cantata on the story of Kitezh that predated Rimsky-Korsakov's opera on the same subject. The concerto was variously extracts from Springtime suite for flute & orchestra (Op. 138) or this Op. 136 Horn Concerto. in one concert (the third one), a movement from the Concerto-Poem for trumpet & orchestra (or so I think) was played before the Horn Concerto, and that one is the one from which I extracted the upload here.

Hi Robert - I'm also interested in de Hartmann.  Do you have, or are you aware of, any other non-commercially available recordings?

Christopher, I'm sorry that I don't know more non-commercial recordings of his works than you do. I only got to know the recording of La Fleurette Rouge from your post!

Some one have uploaded it to here:
https://classical-music-online.net/en/production/216297
This proved to be a monstrous 50-minute-long work in 4 movements. The composer only completed the first movement. The middle movements were found in piano reduction form, the last only in sketches.

Any info about a possible broadcast or commercial recording of this piece?

Thank you for this extra version. I didn’t know it exists. I feel the old Thorofon recording is not a good performance, and Ilan Volkov’s performance finally came to rescue many years later. It is good to see another one is available!

Did anyone recorded it? I planned to but totally forgot about it until days after the broadcast…

My friend just alerted me of an 2018 ABC Classic broadcast (https://www.abc.net.au/listen/classic/tonya-lemoh-australian-recital/10369120) of the Australian composer Raymond Hanson’s piano music and Piano Quintet, the pianist of which was Tonya Lemoh, who went on to record Hanson’s piano music for Grand Piano label. But the Quintet was never commercially released. Does any one who record ABC Classics regularly happened to have saved this broadcast?

This album including piano quintet works by Richard Walthew and John McCabe is released today
https://www.prestomusic.com/classical/products/9779896--p-godfrey-i-hodgson-r-walthew-j-mccabe-piano-quintets