I do it all the time (well, quite often)
You will need a decent ANALOGUE capture card...
Then just wire composite out of your Amiga into Composite IN on your Non linear Capture card !!
Bingo.
You cant really do the "other" choice that I use Because you cant separate frames in the "Demo" ...
That is .. Send each frame over a network and build them up on the pc.
This

Is a good ANologue & DIgital capture card if your interested.....
THIS Is the URL