PPG analysis beyond HR at rest

recorder ... would it be possible to include raw ppg and acc here?

Kind of, but that's not really how Recorder is designed. It writes an update of the state of all data every set amount of time, but realistically for developing an algorithm you want something that writes the exact data received (no more, no less).

I believe HRM Accelerometer event recorder already does this? However if your question is 'could we write all the data to a file on the Bangle so you don't need a PC/Phone within range' then yes, we could possibly do that. I think the reason it wasn't done is the file gets big and then you have to wait to download it between data collections. Normally it's not a hugely big deal to just have a phone on, even during a run.