I’m with vade – build it in Max and see if that works well enough for you – don’t discard the simple solution just because a more complicated one exists… and if you’re not sure if something will work, TRY IT.
Using movies if you want a video sampler is never going to be perfect – yes it will enable otherwise impossible durations but, as you note, you’ll never be able to scrub to and from the present, as the currently recording movie will be unplayable.
If you want to automate it, Syphon Recorder responds to cmd-alt-R to start/stop recording even when it isn’t frontmost, so if you can generate that keystroke (other threads here describe ways to do that, or perhaps there’s a Max object to generate keystrokes?) then you can automate Recorder. I have done custom OSC-enabled builds for people before – get in touch if that’s of interest, but do try to build your own solution first – it might work better.