Hi Peter, I just wanted to share my project here, in case it helps: https://github.com/samesimilar/m5_soundfile/
This is an attempt to implement versions of readsf~ and writesf~ that can start/stop on sample-accurate (internal-to-pd) timing (including in the 'past' via a buffer). This means that you should be able to schedule recording or playback within blocks at specific samples.
(I wrote these externals to enable more predictably synchronized looping on the Critter & Guitari '5 Moons' looper device.)