Isolating track after ballasting?

[torikoos]

Inserting railjoiners without disturbing the track is pretty tricky, however if your rail is laid down properly, then just cutting the rail with your mentioned saw is good enough. The rails won't move much, unless your train room has excessive temperature fluctuations, and by cutting through you've already created the electric separation.
I you're not entirely sure, you could insert a tiny piece of styrene and carefully sand/file that to the right shape. A drop of white glue will do the same, but may be a little trickier to shape.

Koos