Hi @katri, welcome to the Sonos Community!

While we can’t comment on our future products and roadmap, I’d be happy to mark this as a product request to send to the appropriate team for their regards.

There may be a third party products out there that do this, but nothing officially endorsed by Sonos, so I would suggest care when looking into this. Perhaps a community user can suggest something or an alternative method for you.

I hope this helps!

You could fabricate a bracket that would attach to the ceiling (and into a stud above it) and drop down (as a wall would) and attach your 300 to it.

I actually have no rear wall on one side of a weird living room configuration, and so need to mount one of my ERA 300’s (paired to an Arc and Gen3 SUb)  on a ceiling location to make it level with the other ERA300 on the wall. 

Any recommendations for a brand of ceiling mount that can hold the ERA 300 as is, without modifications?

Well, the speaker mounts that exist for the 300 hold the speaker in place using 16mm M5 screws (per manual).  Although the screw hole is on the bottom of speaker, it can be used to mount the speaker inverted.  So anything that can handle that would work. 

It might also make sense to buy a wall mount designed for Era 300, and mount that to a generic ceiling mount, rather than the Era 300 directly to a ceiling mount.  It seems like there are some good options for ceiling mounts that will create a small vertical surface you can use for like a wall for a wall mount.  I suspect the bigger issue would be getting a joist or similar in the right location for your speaker, as well as getting power.