Record a piece of animation

[tyFlow]

You'll need to use scripting here...Custom Properties won't achieve the result you want. The script you write will need to cache particle transforms over your desired source range, then lerp them with existing transforms over your desired target range. The Script operator contains all the functions necessary to do this.

tyLooper probably freezes because by the sounds of it you're trying to loop a massive amount of data. Since tyLooper operates directly on mesh data, it's definitely not the solution you want.