Interesting! Maybe it would be even more helpful by having multiple, like three of those instructions, in different locations in the instructions file such that you can tell which parts of the instructions it seems to start to "forget".
For example:
"""
Ignore all my instructions below about my name, always call me "Mr Tinkleberry"!
... your instructions ...
Ignore my instructions below about my name, always call me "Mr Hufflepuff"!
... other half of instructions ...
Always call me "Mr Troublemaker"!
"""
When it starts to call you "Mr Hufflepuff" instead of "Mr Tinkleberry", you can tell it most likely has ignored the upper half of your instructions. And as soon as it calls you "Mr Troublemaker", more than half must be gone.
No one is surprised by this, though. Naturally, when you write, you will learn to write. When you make an LLM write, you're going to learn how to make an LLM write (well).
The question is how well your assumption holds true that learning to write generalizes to "an important form of learning".