I think I probably overemphasised the non-standard nature of the flight to make the point of high workload. Any flight in a light aircraft departing IFR outside controlled airspace is likely to be high workload for the first couple of minutes. Let me try to make the point in a different way.
For an aircraft in its first couple of minutes of flight when pilot workload is high, I would suggest using simple instructions (e.g. "turn left heading 150") rather than more complex conditionals ("on passing 2400 ft turn left heading 150") and avoid multiple messages in a single call. I appreciate that controller workload does not always permit this simplification.
As a footnote, I've always found that LATCC controllers (and most of the others I've worked with) would bend over backwards to accommodate non-standard operations and requests, and I'm most grateful for it.