So when I ask Siri to play RP, s/he tells me "provided by TuneIn." 
I don't have a TuneIn account of any kind, I think, but do I need to start figuring that out to get Siri to play FLAC?

TuneIn doesn't normally allow you to add FLAC URLs (just AAC or MP3). However, a clever listener emailed me this workaround:

Just found a hack to add the FLAC stream to TuneIn - I’m listening to it now on the Linn system. Essentially, one adds the FLAC stream to one’s TuneIn Favourite station on the iPhone then selects it to play from one’s HiFi system controller. Here’s what I did:

1. Opened my iPhone TuneIn app
2. Logged into my TuneIn account
3. Selected the “Library” tab
4. Picked the “Custom URL” button
5. Put in the URL for your FLAC stream (with metadata)
6. Even though there was a problem playing it on the iPhone (doesn’t support FLAC), I clicked the Add to Favourites i.e. the Heart symbol
7. Waited a couple of hours
8. Went onto my Linn Control app and there was the FLAC stream in my list of TuneIn favourites
9. Selected the FLAC stream and it plays on the Linn system

I haven't tested this myself, as I don't currently have any TuneIn-based hardware players. I'd be curious to see followup posts from anyone who wants to give it a try with players like Bose, etc. 

Click here for the list of stream links, including FLAC-with-metadata for all channels.