How can I trigger a CLIENT_DATA event manually?

I don't think you need an explicit buffer. The data is implicitly buffered by BIG-IP and is accessible with TCP::payload. If the terminator is split across two or more TCP segments, then you simply need to scan TCP::payload for the complete terminator on each CLIENT_DATA event. Your iRule is operating on the TCP payload. There's not much need to be concerned about packets and segmentation.