Percussion line in middle of multi-staff instrument

Hi there! Is there a way to add a percussion line in the middle of a multi-staff instrument? (See example below)
Want to achieve.png
Adding an extra staff adds it either above or below both, but not in the middle; also, it’s a five line staff. Adding an additional instrument to the player results in something similar, with the exception that this time I can use a percussion line.

The workaround I’ve come up with is adding two instruments to the player: an unpitched percussion one and a duplicate of the original (e.g. piano, harp). Then, I hide the lower staff of the original instrument and hide the upper staff of the new multi-staff instrument. Finally, in engrave mode, I arrange the brackets and the spacing. (See example below)

BUUUUUUT… This messes up many layout options, as well as the score layout. Also, for some unexplainable reason, I can’t seem to modify the space between the first instrument and the percussion line. (See image below; also, I’m attaching that example project).

Here’s the example project. (429 KB)

There is no way to add a single-line percussion instrument between the two staves of a grand staff indtrument. The approach you’re taking is the only one I can think of. When adjusting the staff spacing, the offset you apply is being applied to the first staff of each instrument held by the player.

Oh, I understand. Thanks for the answer.
So, there’s no way to adjust the distance between the first instrument and the percussion line (in the example above); am I understanding it right?

You are understanding it right, at least for now.

Thank you Daniel for your precious time.